layer 4 port assignment

Primary TCP/IP Port Assignments and Descriptions

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The OSI Model

What is the osi model.

How a single bit travels from one computer to the next is a complex concept. In 1984, the open systems interconnection (OSI) model was published as a framework for network communication. The model breaks down computer network communication into seven layers. All of the layers work together to create a digital message. The message is built as it moves down the protocol stack. However, it is not sent to another network until it reaches the physical layer.

The model helps IT, computer science, and cybersecurity professionals understand how a single bit travels from one computer to the next by breaking the system into these layers.

From physical devices to user interfaces (UI), this model explains the communication role of each layer in overall computer networking. This article will start by introducing the Physical Layer (Layer 1).

Layer 1: the physical layer

The physical layer is where data moves across network interfaces as digital signals. Additionally, this is where the transmitting and receiving of network communication occurs. Starting with the Application Layer the message moves down the OSI model, and it eventually reaches the Physical Layer for transmission. When the message is received by the physical layer, the message will then move up the OSI layers until it reaches the final application layer.

Layer 2: data-link layer

Electrical signals received (or transmitted) to the physical layer are linked and translated to digital logic in the data-Link layer . Computer devices may be networked at the Data-Link layer, but only as a Local Area Network (LAN). Connecting a LAN to another LAN occurs at Layer 3.

Within Layer 2, the Protocol Data Unit (PDU) known as a frame consists of a header, footer, and data. Understanding how a frame is structured is important for network traffic analysis.

Additionally, within Layer 2, physical addresses are assigned and are also known as MAC addresses and/or hardware addresses in networking. MAC addresses are unique to each device on a local network. They are 48-bits in length and are assigned in hexadecimal characters.

Some other things to note about Layer 2 is that there are a few protocols that reside in it that we should know about:

• Ethernet : The most common type of LAN, Ethernet is the standard used to connect computing devices, routers, and switches in a wired network.
• IEEE 802.11 : “Wi-Fi” or “Wireless LAN.”
• Fiber Distributed Data Interface (FDDI) : Network standard for fiber optic LAN connections.
• Link Layer Discovery Protocol (LLDP) : A Link Layer protocol used for advertising neighbors, identity, and capabilities on a LAN.
• Address Resolution Protocol (ARP) : Converts and links Internet Protocol (IP) addresses to MAC addresses on a LAN.
• Cisco Discovery Protocol (CDP) : Similar to LLDP, but Cisco proprietary. The protocol collects neighbor information of directly connected LAN devices.

Additionally, Layer 2 is split into two sublayers:

• Logical Link Control (LLC) : Responsible for establishing the logical link between devices on a local network.
• Media Access Control (MAC) : Responsible for the procedures used by devices across a network medium.

Layer 3: network layer

When we think of the internet, we are thinking of interconnected networks. Interconnecting networks refer to a Local Area Network (LAN) connection to neighboring or remote networks. Layer 3 of the OSI model, the network layer , is where internetworking takes place and is where logical addresses are assigned to networked devices. A primary function of this layer is to route network packets from one LAN to another. Routing requires IP addresses and logical mapping of other networks across the internet to properly deliver messages. Another important function of Layer 3 is its ability to fragment and reassemble large communication. When Layer 3 passes a message down to Layer 2 for transmission, message length limits may be encountered in some cases.

Additionally, Layer 3 is the layer where the protocols used to route communication between networks reside. A few common network protocols are:

• Internet Protocol (IP) : IPv4 and IPv6 are two versions of IP, and IPv4 is the most common protocol of the Internet .
• Internet Protocol Secure (IPSec) : A more secure version of IP which leverages cryptography.
• Routing Information Protocol (RIP) : Distance-vector routing protocol that uses hop count as a metric of routing.
• Enhanced Interior Gateway Routing Protocol (EiGRP) : Cisco proprietary. A distance-vectoring protocol used for automating network configurations and routing decisions.
• Internet Control Message Protocol (ICMP) : Network protocol used for error reporting of network issues.
• Border Gateway Protocol (BGP) : A routing protocol designed to exchange routing information automatically on the internet.

Within Layer 3, the Protocol Data Unit (PDU) is the packet . Packets encapsulate data intended for transmission with header and footer data.

The IPv4 protocol encapsulates data with IPv4 header information necessary for delivery. For example, the 32-bit packet format contains the source address, the destination address, protocol, time-to-live (TTL), etc. in the IPv4 header data.

Layer 4: transport layer

The transport layer , Layer 4, is responsible for being the go-between the abstract layers of the OSI model (Layers 7-5) and the concrete communication layers (Layers 3-1).

Depending on the type of application, the transportation of that application’s communication will need to be handled in a specific way. For example, basic web browsing communication uses Hypertext Transfer Protocol (HTTP) . HTTP communicates via a specific connection service type and port. The transport layer is responsible for delivering/receiving the HTTP communication and maintaining the connection throughout the HTTP communication.

The Protocol Data Unit (PDU) at Layer 4 is known as a data segment . Segmentation is the process of dividing raw data into smaller pieces. Once the raw data is packaged from the higher application layers it is segmented at the transport layer before being passed to the Network Layer.

The transport layer protocols are divided into two categories depending on their connection service type:

Connection-oriented services

This connection type establishes a logical connection between two devices prior to beginning communication across a network. Connection-oriented protocols typically maintain service connection by following a set of rules that initiate, negotiate, manage, and terminate the communication. The Transport Layer protocols will also retransmit any data that is received without acknowledgment. The most common Connection-Oriented protocol is the Transmission Control Protocol (TCP) and its process to manage a connection between two devices is called the Three-Way Handshake . In TCP communication, the communicating devices typically share a client/server relationship where a client initiates communication with a service. The handshake involves the process of sending special TCP messages to synchronize a state of negotiated connection in communication.

Connectionless services

In connectionless communication, the protocol does not establish a connection between client and server. Instead, once a request is made to the server, the server sends all data without initiation, negotiation, or management of connection. Connectionless protocols also do not attempt to correct any interruptions in data transmission. Once the server sends the data, the server is not concerned if the client receives it.

When TCP or UDP are used to establish communication, the communication is assigned a port as the Layer 4 address. A port is a logical assignment given to processes and their respective application protocols on a computing system. A few important facts to memorize about ports are:

• There are 65,535 valid port numbers available to assign to a communication process.
• Ports 0 - 1023 are Well-Known Ports : Assigned to universal TCP/IP application protocols. These protocols are the most common such as HTTPS, SSH, FTP, DNS, and the list goes on. They are registered to these protocols by a global
• Ports 1024 - 49,151 are Registered Ports : Reserved for application protocols that are not specified as universal TCP/IP application protocols.
• Ports 49,152 - 65,535 are Private/Dynamic Ports : These ports may be used for any process without the need to register the port with the global assigning authority.
• When TCP and IP are used together, a Layer 4 port and a Layer 3 IP address are assigned to the connection. This is called a socket. For example, 8.8.8.8:443 is a socket indicating that communication to IP address 8.8.8.8 is to connect to port 443 on the server.

Layer 5: session layer

The session layer starts, manages, and terminates sessions between end-user application processes. Sessions are considered the persistent connection between devices. A session is application-focused; sessions are not concerned with layers 1-4. Instead, the session layer controls dialog between two networked devices. It is considered to facilitate host-to-host communication. Sessions dialog may be controlled through synchronization checkpoints, and through management of communication modes. There are two modes of communication permitted at Layer 5:

• Half-Duplex : Communication travels in both directions between sender and receiver, but only one device may transmit a message at a time.
• Full-Duplex : Communication travels in both directions between sender and receiver, and messages may be sent simultaneously in either direction.

The session layer resembles a phone conversation. For example, when a person picks up a phone and calls someone else a session is created. Once the communication on the call is completed, the session is terminated by hanging up the phone. In computing, software applications are making the phone call and establishing a session.

Two common Layer 5 protocols still used today are:

• Remote Procedure Call (RPC)

Layer 6: presentation layer

The presentation layer is primarily responsible for presenting data so that the recipient will understand the data. Data formatting and encoding protocols apply at Layer 6 to ensure data is legible and presented properly in the application receiving it. Data compression is also a function of Layer 6. If necessary, data may be compressed to improve data throughput over network communication.

Some common Layer 6 protocols are ASCII , JPEG , GIF , MPEG , and PNG .

Another main function of the presentation layer is the encryption and decryption of data sent across a network. Most encryption communication protocols straddle multiple layers of the OSI model, but the actual encryption function is Layer 6.

Two of the most common secure communication protocols are:

• Transport Layer Security (TLS)
• Secure Socket Layer (SSL)

Layer 7: application layer

The topmost layer of the OSI model is the application layer . On computer systems, applications display information to the user via the UI.

Note : Software applications running on a computer are NOT considered to reside in the application layer. Instead, they leverage application layer services and protocols that enable network communication.

For example, the user can craft messages and access the network from the application layer. A web browser application allows a user to access a web page. The user may input information and receive information through the web browser. However, the application layer protocol HTTP performs the network communication function. The web browser and HTTP work closely together, and the distinction between the two may be subtle. Yet, HTTP is the web browsing protocol for all web browser applications. In contrast, no single web browser software exclusively utilizes HTTP.

HTTP is one of many common application layer protocols. Below are a few additional protocols to know. It is also good practice to memorize the associated port assigned to the protocols:

The OSI model breaks down computer network communication into seven layers. All of the layers work together to create a digital message. Understanding the OSI model will help you communicate with other network technologists. Computer networking may seem complex, but, with a bit of study, you can gain this knowledge to become an effective Cybersecurity Analyst.

Learn More on Codecademy

Cybersecurity analyst interview prep, code foundations.

  Layer 4 Transport Layer

TCP; UDP; Transporation

The tasks of the transport layer (also end-to-end control, transport control) include the segmentation of the data stream and in relieving congestion.

A data segment is a Service Data Unit, which is used for encapsulation on the fourth layer (transport layer). It consists of protocol elements that contain Layer 4 information control. When addressing the data segment assigned a Layer 4 address, so a port. The data segment is encapsulated in the layer 3 in a data packet.

The transport layer provides the application-oriented layers 5 to 7 standardized access so that they do not need to consider the characteristics of the communications network.

Five different service classes of different grades are defined in layer 4 and may be used by the upper layers, from the simplest to the most comfortable service with multiplex mechanisms, error protection and troubleshooting procedures..

OSI Layer 4 - Transport Layer

In computer networking, the transport layer is a conceptual division of methods in the layered architecture of protocols in the network stack in the Internet Protocol Suite and the Open Systems Interconnection (OSI). The protocols of the layer provide host-to-host communication services for applications.[1] It provides services such as connection-oriented data stream support, reliability, flow control, and multiplexing. The details of implementation and semantics of the Transport Layer of the TCP/IP model (RFC 1122), which is the foundation of the Internet, and the Open Systems Interconnection (OSI) model of general networking, are different. In the OSI model the transport layer is most often referred to as Layer 4 or L4, while numbered layers are not used in TCP/IP. The best-known transport protocol of TCP/IP is the Transmission Control Protocol (TCP), and lent its name to the title of the entire suite. It is used for connection-oriented transmissions, whereas the connectionless User Datagram Protocol (UDP) is used for simpler messaging transmissions. TCP is the more complex protocol, due to its stateful design incorporating reliable transmission and data stream services. Other prominent protocols in this group are the Datagram Congestion Control Protocol (DCCP) and the Stream Control Transmission Protocol (SCTP). Wikipedia

• Connection-oriented communication
• Same order delivery
• Reliability
• Flow control
• Congestion avoidance
• Port Multiplexing

Popular Transport Layer Protocols

Layer 7   Application Layer

Layer 6   presentation layer, layer 5   session layer, layer 4   transport layer, layer 3   network layer, layer 2   data link layer, layer 1   physical layer.

The OSI Model – The 7 Layers of Networking Explained in Plain English

This article explains the Open Systems Interconnection (OSI) model and the 7 layers of networking, in plain English.

The OSI model is a conceptual framework that is used to describe how a network functions. In plain English, the OSI model helped standardize the way computer systems send information to each other.

Learning networking is a bit like learning a language - there are lots of standards and then some exceptions. Therefore, it’s important to really understand that the OSI model is not a set of rules. It is a tool for understanding how networks function.

Once you learn the OSI model, you will be able to further understand and appreciate this glorious entity we call the Internet, as well as be able to troubleshoot networking issues with greater fluency and ease.

All hail the Internet!

Prerequisites

You don’t need any prior programming or networking experience to understand this article. However, you will need:

• Basic familiarity with common networking terms (explained below)
• A curiosity about how things work :)

Learning Objectives

Over the course of this article, you will learn:

• What the OSI model is
• The purpose of each of the 7 layers
• The problems that can happen at each of the 7 layers
• The difference between TCP/IP model and the OSI model

Common Networking Terms

Here are some common networking terms that you should be familiar with to get the most out of this article. I’ll use these terms when I talk about OSI layers next.

A node is a physical electronic device hooked up to a network, for example a computer, printer, router, and so on. If set up properly, a node is capable of sending and/or receiving information over a network.

Nodes may be set up adjacent to one other, wherein Node A can connect directly to Node B, or there may be an intermediate node, like a switch or a router, set up between Node A and Node B.

Typically, routers connect networks to the Internet and switches operate within a network to facilitate intra-network communication. Learn more about hub vs. switch vs. router.

Here's an example:

For the nitpicky among us (yep, I see you), host is another term that you will encounter in networking. I will define a host as a type of node that requires an IP address. All hosts are nodes, but not all nodes are hosts. Please Tweet angrily at me if you disagree.

Links connect nodes on a network. Links can be wired, like Ethernet, or cable-free, like WiFi.

Links to can either be point-to-point, where Node A is connected to Node B, or multipoint, where Node A is connected to Node B and Node C.

When we’re talking about information being transmitted, this may also be described as a one-to-one vs. a one-to-many relationship.

A protocol is a mutually agreed upon set of rules that allows two nodes on a network to exchange data.

“A protocol defines the rules governing the syntax (what can be communicated), semantics (how it can be communicated), and synchronization (when and at what speed it can be communicated) of the communications procedure. Protocols can be implemented on hardware, software, or a combination of both. Protocols can be created by anyone, but the most widely adopted protocols are based on standards.” - The Illustrated Network.

Both wired and cable-free links can have protocols.

While anyone can create a protocol, the most widely adopted protocols are often based on standards published by Internet organizations such as the Internet Engineering Task Force (IETF).

A network is a general term for a group of computers, printers, or any other device that wants to share data.

Network types include LAN, HAN, CAN, MAN, WAN, BAN, or VPN. Think I’m just randomly rhyming things with the word can ? I can ’t say I am - these are all real network types. Learn more here .

Topology describes how nodes and links fit together in a network configuration, often depicted in a diagram. Here are some common network topology types:

A network consists of nodes, links between nodes, and protocols that govern data transmission between nodes.

At whatever scale and complexity networks get to, you will understand what’s happening in all computer networks by learning the OSI model and 7 layers of networking.

What is the OSI Model?

The OSI model consists of 7 layers of networking.

First, what’s a layer?

No, a layer - not a lair . Here there are no dragons.

A layer is a way of categorizing and grouping functionality and behavior on and of a network.

In the OSI model, layers are organized from the most tangible and most physical, to less tangible and less physical but closer to the end user.

Each layer abstracts lower level functionality away until by the time you get to the highest layer. All the details and inner workings of all the other layers are hidden from the end user.

How to remember all the names of the layers? Easy.

• Please | Physical Layer
• Do | Data Link Layer
• Not | Network Layer
• Tell (the) | Transport Layer
• Secret | Session Layer
• Password (to) | Presentation Layer
• Anyone | Application Layer

Keep in mind that while certain technologies, like protocols, may logically “belong to” one layer more than another, not all technologies fit neatly into a single layer in the OSI model. For example, Ethernet, 802.11 (Wifi) and the Address Resolution Protocol (ARP) procedure operate on >1 layer.

The OSI is a model and a tool, not a set of rules.

OSI Layer 1

Layer 1 is the physical layer . There’s a lot of technology in Layer 1 - everything from physical network devices, cabling, to how the cables hook up to the devices. Plus if we don’t need cables, what the signal type and transmission methods are (for example, wireless broadband).

Instead of listing every type of technology in Layer 1, I’ve created broader categories for these technologies. I encourage readers to learn more about each of these categories:

• Nodes (devices) and networking hardware components. Devices include hubs, repeaters, routers, computers, printers, and so on. Hardware components that live inside of these devices include antennas, amplifiers, Network Interface Cards (NICs), and more.
• Device interface mechanics. How and where does a cable connect to a device (cable connector and device socket)? What is the size and shape of the connector, and how many pins does it have? What dictates when a pin is active or inactive?
• Functional and procedural logic. What is the function of each pin in the connector - send or receive? What procedural logic dictates the sequence of events so a node can start to communicate with another node on Layer 2?
• Cabling protocols and specifications. Ethernet (CAT), USB, Digital Subscriber Line (DSL) , and more. Specifications include maximum cable length, modulation techniques, radio specifications, line coding, and bits synchronization (more on that below).
• Cable types. Options include shielded or unshielded twisted pair, untwisted pair, coaxial and so on. Learn more about cable types here .
• Signal type. Baseband is a single bit stream at a time, like a railway track - one-way only. Broadband consists of multiple bit streams at the same time, like a bi-directional highway.
• Signal transmission method (may be wired or cable-free). Options include electrical (Ethernet), light (optical networks, fiber optics), radio waves (802.11 WiFi, a/b/g/n/ac/ax variants or Bluetooth). If cable-free, then also consider frequency: 2.5 GHz vs. 5 GHz. If it’s cabled, consider voltage. If cabled and Ethernet, also consider networking standards like 100BASE-T and related standards.

The data unit on Layer 1 is the bit.

A bit the smallest unit of transmittable digital information. Bits are binary, so either a 0 or a 1. Bytes, consisting of 8 bits, are used to represent single characters, like a letter, numeral, or symbol.

Bits are sent to and from hardware devices in accordance with the supported data rate (transmission rate, in number of bits per second or millisecond) and are synchronized so the number of bits sent and received per unit of time remains consistent (this is called bit synchronization). The way bits are transmitted depends on the signal transmission method.

Nodes can send, receive, or send and receive bits. If they can only do one, then the node uses a simplex mode. If they can do both, then the node uses a duplex mode. If a node can send and receive at the same time, it’s full-duplex – if not, it’s just half-duplex.

The original Ethernet was half-duplex. Full-duplex Ethernet is an option now, given the right equipment.

How to Troubleshoot OSI Layer 1 Problems

Here are some Layer 1 problems to watch out for:

• Defunct cables, for example damaged wires or broken connectors
• Broken hardware network devices, for example damaged circuits
• Stuff being unplugged (...we’ve all been there)

If there are issues in Layer 1, anything beyond Layer 1 will not function properly.

Layer 1 contains the infrastructure that makes communication on networks possible.

It defines the electrical, mechanical, procedural, and functional specifications for activating, maintaining, and deactivating physical links between network devices. - Source

Fun fact: deep-sea communications cables transmit data around the world. This map will blow your mind: https://www.submarinecablemap.com/

And because you made it this far, here’s a koala:

OSI Layer 2

Layer 2 is the data link layer . Layer 2 defines how data is formatted for transmission, how much data can flow between nodes, for how long, and what to do when errors are detected in this flow.

In more official tech terms:

• Line discipline. Who should talk for how long? How long should nodes be able to transit information for?
• Flow control. How much data should be transmitted?
• Error control - detection and correction . All data transmission methods have potential for errors, from electrical spikes to dirty connectors. Once Layer 2 technologies tell network administrators about an issue on Layer 2 or Layer 1, the system administrator can correct for those errors on subsequent layers. Layer 2 is mostly concerned with error detection, not error correction. ( Source )

There are two distinct sublayers within Layer 2:

• Media Access Control (MAC): the MAC sublayer handles the assignment of a hardware identification number, called a MAC address, that uniquely identifies each device on a network. No two devices should have the same MAC address. The MAC address is assigned at the point of manufacturing. It is automatically recognized by most networks. MAC addresses live on Network Interface Cards (NICs). Switches keep track of all MAC addresses on a network. Learn more about MAC addresses on PC Mag and in this article . Learn more about network switches here .
• Logical Link Control (LLC): the LLC sublayer handles framing addressing and flow control. The speed depends on the link between nodes, for example Ethernet or Wifi.

The data unit on Layer 2 is a frame .

Each frame contains a frame header, body, and a frame trailer:

• Header: typically includes MAC addresses for the source and destination nodes.
• Body: consists of the bits being transmitted.
• Trailer: includes error detection information. When errors are detected, and depending on the implementation or configuration of a network or protocol, frames may be discarded or the error may be reported up to higher layers for further error correction. Examples of error detection mechanisms: Cyclic Redundancy Check (CRC) and Frame Check Sequence (FCS). Learn more about error detection techniques here .

Typically there is a maximum frame size limit, called an Maximum Transmission Unit, MTU. Jumbo frames exceed the standard MTU, learn more about jumbo frames here .

How to Troubleshoot OSI Layer 2 Problems

Here are some Layer 2 problems to watch out for:

• All the problems that can occur on Layer 1
• Unsuccessful connections (sessions) between two nodes
• Sessions that are successfully established but intermittently fail
• Frame collisions

The Data Link Layer allows nodes to communicate with each other within a local area network. The foundations of line discipline, flow control, and error control are established in this layer.

OSI Layer 3

Layer 3 is the network layer . This is where we send information between and across networks through the use of routers. Instead of just node-to-node communication, we can now do network-to-network communication.

Routers are the workhorse of Layer 3 - we couldn’t have Layer 3 without them. They move data packets across multiple networks.

Not only do they connect to Internet Service Providers (ISPs) to provide access to the Internet, they also keep track of what’s on its network (remember that switches keep track of all MAC addresses on a network), what other networks it’s connected to, and the different paths for routing data packets across these networks.

Routers store all of this addressing and routing information in routing tables.

Here’s a simple example of a routing table:

The data unit on Layer 3 is the data packet . Typically, each data packet contains a frame plus an IP address information wrapper. In other words, frames are encapsulated by Layer 3 addressing information.

The data being transmitted in a packet is also sometimes called the payload . While each packet has everything it needs to get to its destination, whether or not it makes it there is another story.

Layer 3 transmissions are connectionless, or best effort - they don't do anything but send the traffic where it’s supposed to go. More on data transport protocols on Layer 4.

Once a node is connected to the Internet, it is assigned an Internet Protocol (IP) address, which looks either like 172.16. 254.1 (IPv4 address convention) or like 2001:0db8:85a3:0000:0000:8a2e:0370:7334 (IPv6 address convention). Routers use IP addresses in their routing tables.

IP addresses are associated with the physical node’s MAC address via the Address Resolution Protocol (ARP), which resolves MAC addresses with the node’s corresponding IP address.

ARP is conventionally considered part of Layer 2, but since IP addresses don’t exist until Layer 3, it’s also part of Layer 3.

How to Troubleshoot OSI Layer 3 Problems

Here are some Layer 3 problems to watch out for:

• All the problems that can crop up on previous layers :)
• Faulty or non-functional router or other node
• IP address is incorrectly configured

Many answers to Layer 3 questions will require the use of command-line tools like ping , trace , show ip route , or show ip protocols . Learn more about troubleshooting on layer 1-3 here .

The Network Layer allows nodes to connect to the Internet and send information across different networks.

OSI Layer 4

Layer 4 is the transport layer . This where we dive into the nitty gritty specifics of the connection between two nodes and how information is transmitted between them. It builds on the functions of Layer 2 - line discipline, flow control, and error control.

This layer is also responsible for data packet segmentation, or how data packets are broken up and sent over the network.

Unlike the previous layer, Layer 4 also has an understanding of the whole message, not just the contents of each individual data packet. With this understanding, Layer 4 is able to manage network congestion by not sending all the packets at once.

The data units of Layer 4 go by a few names. For TCP, the data unit is a packet. For UDP, a packet is referred to as a datagram. I’ll just use the term data packet here for the sake of simplicity.

Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) are two of the most well-known protocols in Layer 4.

TCP, a connection-oriented protocol, prioritizes data quality over speed.

TCP explicitly establishes a connection with the destination node and requires a handshake between the source and destination nodes when data is transmitted. The handshake confirms that data was received. If the destination node does not receive all of the data, TCP will ask for a retry.

TCP also ensures that packets are delivered or reassembled in the correct order. Learn more about TCP here .

UDP, a connectionless protocol, prioritizes speed over data quality. UDP does not require a handshake, which is why it’s called connectionless.

Because UDP doesn’t have to wait for this acknowledgement, it can send data at a faster rate, but not all of the data may be successfully transmitted and we’d never know.

If information is split up into multiple datagrams, unless those datagrams contain a sequence number, UDP does not ensure that packets are reassembled in the correct order. Learn more about UDP here .

TCP and UDP both send data to specific ports on a network device, which has an IP address. The combination of the IP address and the port number is called a socket.

Learn more about sockets here .

Learn more about the differences and similarities between these two protocols here .

How to Troubleshoot OSI Layer 4 Problems

Here are some Layer 4 problems to watch out for:

• Blocked ports - check your Access Control Lists (ACL) & firewalls
• Quality of Service (QoS) settings. QoS is a feature of routers/switches that can prioritize traffic, and they can really muck things up. Learn more about QoS here .

The Transport Layer provides end-to-end transmission of a message by segmenting a message into multiple data packets; the layer supports connection-oriented and connectionless communication.

OSI Layer 5

Layer 5 is the session layer . This layer establishes, maintains, and terminates sessions.

A session is a mutually agreed upon connection that is established between two network applications. Not two nodes! Nope, we’ve moved on from nodes. They were so Layer 4.

Just kidding, we still have nodes, but Layer 5 doesn’t need to retain the concept of a node because that’s been abstracted out (taken care of) by previous layers.

So a session is a connection that is established between two specific end-user applications. There are two important concepts to consider here:

• Client and server model: the application requesting the information is called the client, and the application that has the requested information is called the server.
• Request and response model: while a session is being established and during a session, there is a constant back-and-forth of requests for information and responses containing that information or “hey, I don’t have what you’re requesting.”

Sessions may be open for a very short amount of time or a long amount of time. They may fail sometimes, too.

Depending on the protocol in question, various failure resolution processes may kick in. Depending on the applications/protocols/hardware in use, sessions may support simplex, half-duplex, or full-duplex modes.

Examples of protocols on Layer 5 include Network Basic Input Output System (NetBIOS) and Remote Procedure Call Protocol (RPC), and many others.

From here on out (layer 5 and up), networks are focused on ways of making connections to end-user applications and displaying data to the user.

How to Troubleshoot OSI Layer 5 Problems

Here are some Layer 5 problems to watch out for:

• Servers are unavailable
• Servers are incorrectly configured, for example Apache or PHP configs
• Session failure - disconnect, timeout, and so on.

The Session Layer initiates, maintains, and terminates connections between two end-user applications. It responds to requests from the presentation layer and issues requests to the transport layer.

OSI Layer 6

Layer 6 is the presentation layer . This layer is responsible for data formatting, such as character encoding and conversions, and data encryption.

The operating system that hosts the end-user application is typically involved in Layer 6 processes. This functionality is not always implemented in a network protocol.

Layer 6 makes sure that end-user applications operating on Layer 7 can successfully consume data and, of course, eventually display it.

There are three data formatting methods to be aware of:

• American Standard Code for Information Interchange (ASCII): this 7-bit encoding technique is the most widely used standard for character encoding. One superset is ISO-8859-1, which provides most of the characters necessary for languages spoken in Western Europe.
• Extended Binary-Coded Decimal Interchange Code (EBDCIC): designed by IBM for mainframe usage. This encoding is incompatible with other character encoding methods.
• Unicode: character encodings can be done with 32-, 16-, or 8-bit characters and attempts to accommodate every known, written alphabet.

Learn more about character encoding methods in this article , and also here .

Encryption: SSL or TLS encryption protocols live on Layer 6. These encryption protocols help ensure that transmitted data is less vulnerable to malicious actors by providing authentication and data encryption for nodes operating on a network. TLS is the successor to SSL.

How to Troubleshoot OSI Layer 6 Problems

Here are some Layer 6 problems to watch out for:

• Non-existent or corrupted drivers
• Incorrect OS user access level

The Presentation Layer formats and encrypts data.

OSI Layer 7

Layer 7 is the application layer .

True to its name, this is the layer that is ultimately responsible for supporting services used by end-user applications. Applications include software programs that are installed on the operating system, like Internet browsers (for example, Firefox) or word processing programs (for example, Microsoft Word).

Applications can perform specialized network functions under the hood and require specialized services that fall under the umbrella of Layer 7.

Electronic mail programs, for example, are specifically created to run over a network and utilize networking functionality, such as email protocols, which fall under Layer 7.

Applications will also control end-user interaction, such as security checks (for example, MFA), identification of two participants, initiation of an exchange of information, and so on.

Protocols that operate on this level include File Transfer Protocol (FTP), Secure Shell (SSH), Simple Mail Transfer Protocol (SMTP), Internet Message Access Protocol (IMAP), Domain Name Service (DNS), and Hypertext Transfer Protocol (HTTP).

While each of these protocols serve different functions and operate differently, on a high level they all facilitate the communication of information. ( Source )

How to Troubleshoot OSI Layer 7 Problems

Here are some Layer 7 problems to watch out for:

• All issues on previous layers
• Incorrectly configured software applications
• User error (... we’ve all been there)

The Application Layer owns the services and functions that end-user applications need to work. It does not include the applications themselves.

Our Layer 1 koala is all grown up.

Learning check - can you apply makeup to a koala?

Don’t have a koala?

Well - answer these questions instead. It’s the next best thing, I promise.

• What is the OSI model?
• What are each of the layers?
• How could I use this information to troubleshoot networking issues?

Congratulations - you’ve taken one step farther to understanding the glorious entity we call the Internet.

Learning Resources

Many, very smart people have written entire books about the OSI model or entire books about specific layers. I encourage readers to check out any O’Reilly-published books about the subject or about network engineering in general.

Here are some resources I used when writing this article:

• The Illustrated Network, 2nd Edition
• Protocol Data Unit (PDU): https://www.geeksforgeeks.org/difference-between-segments-packets-and-frames/
• Troubleshooting Along the OSI Model: https://www.pearsonitcertification.com/articles/article.aspx?p=1730891
• The OSI Model Demystified: https://www.youtube.com/watch?v=HEEnLZV2wGI
• OSI Model for Dummies: https://www.dummies.com/programming/networking/layers-in-the-osi-model-of-a-computer-network/

Chloe Tucker is an artist and computer science enthusiast based in Portland, Oregon. As a former educator, she's continuously searching for the intersection of learning and teaching, or technology and art. Reach out to her on Twitter @_chloetucker and check out her website at chloe.dev .

Read more posts .

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Transport Layer of OSI Model (Layer-4)

Transport Layer is the fourth layer in 7 Layer OSI Model after Network Layer. Similar to Layer-2 and Layer-3, this layer also performs addressing & multiplexing but in different domain through TCP and UDP.

OSI Model divides the network communication processes into seven layers in order to simplify it. Each layer performs specific functions to support the layers above it. The seven Layer model starts from Physical till Application Layer & Transport layer is in the middle. The core concept behind Transport layer is the “support of Multitasking”. It allows same computer, browser & internet connection to work on multiple applications simultaneously & this is achieved through Port Numbers, Transport Layer Addressing & Multiplexing. Below figure shows the position of Data Link layer in the OSI Model :

Transport layer is in the middle of the OSI Model as in below figure. It is a part of both the lower and upper of layer groups.

Lower layers, because it is involves the transport of data,

Upper Layers, because its functions are also somewhat high-level.

PDU at Transport Layer is called Segment .

Functions/Duties of Transport Layer

Each Layer in OSI Model Performs some important duties. Important functions performed by Transport Layer are listed here:

• Sequencing: Sequencing is a connection-oriented service that takes TCP segments that are received out of order and place them in the right order
• Error Control: Is uses error control mechanisms to ensure reliable delivery of data. Due to this acknowledgement mechanisms, the receiver can detect how many bits have been corrupted during transmission. Receiver then requests the sender to send those bits again to ensure that no data is lost during transmission.
• Other Functions of Transport Layer include: End to End Connection Management, Transmission, Segmentation and Flow Control
• Transport Layer is responsible for Layer-4 Addressing which is also called Process Level Addressing. It allows a computer to use multiple network layer protocols simultaneously.

Transport Layer also performs Multiplexing and De-multiplexing of data to allow multiple programs to run on same computer using different Port numbers. In modern multi-tasking environments, many network applications need to run on a computer simultaneously. So, there should be some mechanism to identify which application should receive the incoming data.

To make this work correctly, incoming data from different applications is multiplexed at the Transport layer and sent to the Media layers. On the other side of the communication, the data received from the Media layers are de-multiplexed at the Transport layer and delivered to the correct application. This is achieved by using Layer4 “Port Numbers”.

The range of Transport Layer Port numbers is from 0-65,535. 65000 , because Port number is a 16-bit number & maximum range through which it can span is 65,535.

The port numbers are divided into three ranges:

These are some Famous & Well Known Port Numbers:

Transport Layer Protocols

The OSI Model provides a conceptual framework for communication between computers, but the model itself is not a method of communication. Actual communication is made possible by using communication protocols. Each layer on the OSI Model has some protocols associated with it. Some important protocols on Transport layer are listed in below:

• TCP (Transmission Control Protocol)
• UDP (User Datagram Protocol)
• Fiber Channel Protocol
• HSRP, VRRP, …

Network Equipment/Components at Transport Layer

• Load Balancers

It is the 4th Layer in OSI seven layered Model . It performs important functions like Layer-4 Addressing, Multiplexing, Demultiplexing, Sequencing, Error Control, End to End Connection Management, Transmission, Segmentation & Flow Control. Important Protocols at Transport Layer include TCP, UDP, SPX, BGP, ESP, Fiber Channel Protocol, iSCSI and SCTP. Equipment operating at Transport Layer include Firewalls, Gateways and Load Balancers. PDU is called Segment.

Now, test your knowledge on OSI Model using our free Quizzes & Cheat sheet resources for long term memory:

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Written by  Arman Hasen

Home | TCP/IP Essentials | Transport Layer | TCP and UDP

Transport Layer (Layer 4) - Reserved TCP and UDP Port Numbers

* Real Audio's rtsp (real-time streaming protocol) via port 554 is a good example.

Layer 4 Switch

Last Edited

Essentially, a Layer 4 Switch is a Layer 3 switch that is capable of examining layer 4 of each packet that it switches. In TCP/IP networking , this is equivalent to examining the Transmission Control Protocol ( TCP ) layer information in the packet.

Vendors tout Layer 4 switches as being able to use TCP information for prioritizing traffic by application. For example, to prioritize Hypertext Transfer Protocol ( HTTP ) traffic, a Layer 4 switch would give priority to packets whose layer 4 (TCP) information includes TCP port number 80, the standard port number for HTTP communication.

Some vendors foresee higher-layer switches that examine layer 5, 6, or 7 information to provide more control over prioritizing application traffic, but this might be just vendor hype.

What is Layer 4 Switch?

The Layer 4 switch, often referred to as a “layer 3 switch with enhancements” or a “layer 3 switch that understands layer 4 protocols,” represents an advanced breed of network switch. While traditional switches operate at the data link layer (Layer 2) of the OSI model , the Layer 4 switch extends its purview to the transport layer. It goes beyond merely analyzing the source and destination MAC addresses and ventures into the examination of port numbers and specific transport-layer protocols such as TCP and UDP.

This heightened capability allows the Layer 4 switch to provide more granular control over network traffic, enabling functions like Quality of Service (QoS) , traffic prioritization, and security enhancements. The Layer 4 switch’s unique understanding of both network and transport layer information enables more efficient routing decisions and facilitates complex network management tasks that go beyond mere packet switching. It is a critical component in building modern, intelligent, and responsive networks.

Load Balancing

A cloud data center, such as a Google or Microsoft data center, provides many applications concurrently, such as search, email, and video applications. To support requests from external clients, each application is associated with a publicly visible IP address to which clients send their requests and from which they receive responses. Inside the data center, the external requests are first directed to a load balancer whose job it is to distribute requests to the hosts, balancing the load across the hosts as a function of their current load.

A large data center will often have several load balancers, each one devoted to a set of specific cloud applications. Such a load balancer is sometimes referred to as a “layer-4 switch” since it makes decisions based on the destination port number (layer 4) as well as destination IP address in the packet. Upon receiving a request for a particular application, the load balancer forwards it to one of the hosts that handles the application. (A host may then invoke the services of other hosts to help process the request.)

When the host finishes processing the request, it sends its response back to the load balancer, which in turn relays the response back to the external client. The load balancer not only balances the work load across hosts, but also provides a NAT-like function, translating the public external IP address to the internal IP address of the appropriate host, and then translating back for packets traveling in the reverse direction back to the clients.

This prevents clients from contacting hosts directly, which has the security benefit of hiding the internal network structure and preventing clients from directly interacting with the hosts.

• Layer 3 Switch
• Layer 2 – Data-Link Layer

Layer 4 port assignment

A rational number is any number that can be made by dividing oneinteger by another.0.5 is a rational number as it can be made by dividing the number 1by the number 22 is a rational number because it can be made by dividing 2 by 1-6.6 is a rational number because it can be made by dividing -66 by10---------------------------------------------------------Note there are number that are called Irrational Numbers .Irrational numbers are all "real" numbers (numbers with a decimalpoint) that cannot be written as a simple fraction - the decimalgoes on forever without repeating.For instance the number Pi is an irrational number.A rational number is a real number that can be expressed as a ratio of two integers. Another way to think about it is this: if you can write a number as a fraction then it's a rational number.

When data is sent across a network it is encapsulated at different layers of the OSI model. Mainly layer 2, 3, and 4. Layers 2 and 3 are intra and inter network data respectively. At layer 2 data is a frame and has a header that tells which type of media the frame will be transmitted over and a trailer that tells the receiving device if data has been corrupted. At layer 3 you have your IP addresses and network routing information, data here is called a packet. There is only a header at layer 3. Layer 4 is transport which encapsulates data as a Segment. This is where port assignments are important. Layer 4 ports are not to be confused with Layer 1 interfaces which are often also called ports, they are virtual ports that are used by the NIC to determine which protocol will handle the incoming data. From port 1 - 1023 are your well known ports usually used by servers, from 1024 to 49151 are registered ports and are opened by your machine as needed for the applicable services. They aren't as static as the Layer 3 ports but it makes it more difficult to know exactly which parts of your computer are open by assigning them custom assignments. The last set of ports, 49151 and up are your dynamic ports, which I don't know all that much about. The important thing to know about Layer 4 is the difference between TCP and UDP. Whether or not there is a connection between the two end devices and what kind of recovery the machines will perform in transmitting data determines whether or not the port is a UDP or TCP port. Data in forms of web pages and e-mails need to be delivered in full and without error in order for users to have the full experience and so that important information isn't lost in translation. These types of transmissions are handled with TCP or Transmission Control Protocol. In this case a connection is established and each frame is counted and reassembled in the correct order. In cases where data is corrupted at layer 2 and dropped the receiving device will tell the other device that it is missing important information and the sending device will retransmit. Because of the time that this takes segments in the form of videos and phone calls which experience a constant stream of data and are continuously open to segment loss TCP is not desirable. This is where UDP comes into play. Unreliable Delivery Protocol ports simply take data in as it goes and send it to the appropriate programming in what is called "Best effort" transmission. This means that missing sequence numbers are not retransmitted and there is no logical connection between the server sending the information and host receiving. The reason why assigning ports is important is because each port coordinates with its own protocol, or set of rules for handling the segments. You don't want e-mails which are handled and reassembled by TCP being pushed through a UDP port to a protocol that simply handles video streaming, nothing would happen. So assigning port numbers is important for a number of reasons, the most important being telling your computer how to handle the data that it is receiving. On your personal machine and company servers you may assign port numbers that are not well known to other devices, you may not want HTTP to run on just port 80, and you may not want mail going through the common 110 or 25 because you don't want intruders to know which ports they will have direct access to by default. Ultimately your computer will know which ports to assign, and in client mode (asking for data) it will automatically open a random port for the server to send data to. The server will usually have well known ports running because it's easier for client devices to figure out where to go to ask for the data that you need.

to identify the processes or services that are communicating within the end devices

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Are there any transport layer protocol without the concept of port?

Are there any transport layer protocol that does not have the concept of ports? If so what do they use instead? Also how does NAT work for such protocols?

• transport-protocol

2 Answers 2

Yes. There are many layer-4 protocols. You can get the full registered list at Assigned Internet Protocol Numbers .

There are things like IGP protocols, e.g. EIGRP or OSPF transport protocols that do not use port numbers. Port numbers are addresses for some transport protocols. Most of the registered transport protocols do not use port numbers. Some use other addressing, and some do not use any addressing.

The port addressing allows an OS to multiplex the protocol, but many transport protocols do not need to multiplex, or they use something other than port addresses to multiplex. There may be only one application that needs the data being transported by a protocol. With something like TCP, you will have many applications using it for communications, but you may have a transport protocol dedicated to a single application.

There are several version of NAT. The common NAT is really NAPT . NAPT really only works well* for TCP, UDP, and ICMP. Other protocols have real problems with NAPT. Remember that NAPT is only a kludge to extend the life of IPv4 until IPv6 is ubiquitous. The IP paradigm is end-to-end, where each endpoint has a unique address, and NAPT breaks this IP paradigm.

*There are also applications using TCP or UDP that have real problems with NAT.

4.1.2. Network Address Port Translation (NAPT) NAPT extends the notion of translation one step further by also translating transport identifier (e.g., TCP and UDP port numbers, ICMP query identifiers). This allows the transport identifiers of a number of private hosts to be multiplexed into the transport identifiers of a single external address. NAPT allows a set of hosts to share a single external address. Note that NAPT can be combined with Basic NAT so that a pool of external addresses are used in conjunction with port translation. For packets outbound from the private network, NAPT would translate the source IP address, source transport identifier and related fields such as IP, TCP, UDP and ICMP header checksums. Transport identifier can be one of TCP/UDP port or ICMP query ID. For inbound packets, the destination IP address, destination transport identifier and the IP and transport header checksums are translated. A NAPT router in figure 2 may be configured to translate sessions originated from N-Pri into a single external address, say Addr-i. Very often, the external interface address Addr-Nx of NAPT router is used as the address to map N-Pri to.

Yes there is ICMP which did not have the concept of port number , and instead use the ICMP ID just like the port number and for more details you could see the below RFC:

NAT Behavioral Requirements for ICMP

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CCNA 1 Ch 2

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Service Name and Transport Protocol Port Number Registry

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4.7.2 Packet Tracer – Connect the Physical Layer – Instructions Answer

4.7.2 packet tracer – connect the physical layer instructor version, 4.7.1 packet tracer – connect the physical layer instructor version.

Part 1: Identify Physical Characteristics of Internetworking Devices Part 2: Select Correct Modules for Connectivity Part 3: Connect Devices Part 4: Check Connectivity

In this activity, you will explore the different options available on internetworking devices. You will also be required to determine which options provide the necessary connectivity when connecting multiple devices. Finally, you will add the correct modules and connect the devices.

Note: Scoring for this activity is a combination of Packet Tracer-automated scoring and your recorded answers to the questions posed in the instructions. See the Error! Not a valid bookmark self-reference. at the end of this activity and consult with your instructor to determine your final score.

Part 1: Identify Physical Characteristics of Internetworking Devices

Step 1: identify the management ports of a cisco router..

a. Click the East router. The Physical tab should be active.

b. Zoom in and expand the window to see the entire router.

AUX and Console ports

Step 2: Identify the LAN and WAN interfaces of a Cisco router.

There are 2 WAN interfaces and 2 Gigabit Ethernet interfaces.

The output verifies the correct number of interfaces and their designation. The vlan1 interface is a virtual interface that only exists in software.

1000000 Kbit

Step 3: Identify module expansion slots.

5 slots are available

Part 2: Select Correct Modules for Connectivity

Step 1: determine which modules provide the required connectivity..

a. Click East and then click the Physical tab. On the left, beneath the Modules label, you see the available options to expand the capabilities of the router. Click each module. A picture and a description display at the bottom. Familiarize yourself with these options.

HWIC-4ESW module

PT-SWITCH-NM-1FGE

Step 2: Add the correct modules and power up devices.

a. Click East and attempt to insert the appropriate module from Step 1a. Modules are added by clicking the module and dragging it to the empty slot on the device.

The Cannot add a module when the power is on message should display. Interfaces for this router model are not hot-swappable. The device must be turned off before adding or removing modules. Click the power switch located to the right of the Cisco logo to turn off East. Insert the appropriate module from Step 1a. When done, click the power switch to power up East.

Note: If you insert the wrong module and need to remove it, drag the module down to its picture in the bottom right corner, and release the mouse button.

b. Using the same procedure, insert the module that you identified in Step 1b into the empty slot farthest to the right in Switch2.

c. Use the show ip interface brief command on Switch2 to identify the slot in which the module was placed.

GigabitEthernet5/1

Part 3: Connect Devices

This may be the first activity you have done where you are required to connect devices. Although you may not know the purpose of the different cable types, use the table below and follow these guidelines to successfully connect all the devices:

a. Select the appropriate cable type. b. Click the first device and select the specified interface. c. Click the second device and select the specified interface. d. If you have correctly connected two devices, you will see your score increase.

Example: To connect East to Switch1, select the Copper Straight-Through cable type. Click East and choose GigabitEthernet0/0. Then, click Switch1 and choose GigabitEthernet0/1. Your score should now be 4/55.

Note: For the purposes of this activity, link lights are disabled.

Part 4: Check Connectivity

Step 1: check the interface status on east..

a. Click the CLI tab and enter the following commands:

Compare the output to the following:

If all of the cabling is correct the outputs should match.

Step 2: Connect wireless devices, Laptop and TabletPC.

a. Click the Laptop and select the Config Tab. Select the Wireless0 interface. Put a check in the box labeled On next to Port Status. Within a few seconds the wireless connection should appear.

b. Click the Desktop tab of the Laptop. Click on the Web Browser icon to launch the web browser. Enter www.cisco.pka in the URL box and click Go. The page should display Cisco Packet Tracer.

c. Click the TabletPC and select the Config Tab. Select the Wireless0 interface. Put a check in the box labeled On next to Port Status. Within a few seconds the wireless connection should appear.

d. Repeat the steps in Step 2b to verify the page displays.

Step 3: Change the access method of the TabletPC.

a. Click the TabletPC and select the Config Tab. Select the Wireless0 interface. Uncheck the box labeled On next to Port Status. It should now be clear and the wireless connection will drop.

b. Click the 3G/4G Cell1 interface. Put a check in the box labeled On next to Port Status. Within a few seconds the cellular connection should appear.

c. Repeat the process of verifying web access.

Note: You should not have both the wireless0 interface and 3G/4G Cell1 interfaces active at the same time. This may cause confusion to the device when attempting to connect to some resources.

Step 4: Check connectivity of the other PCs.

All of the PCs should have connectivity to the web site and each other. You will learn to use connectivity testing in many upcoming labs.

Download Packet Tracer (.pka) file:

4.7.1 packet tracer - connect the physical layer.pka 535.81 kb 6058 downloads.

Previous Lab 4.7.1 Packet Tracer – Physical Layer Exploration – Physical Mode

Next Lab 9.1.3 Packet Tracer – Identify MAC and IP Addresses

what is the version of this packet tracer it says it is not compatible with my packet tracer

I think the answer for the first question in Step 3: Identify module expansion slots should be=2. please check

I thing the secound one is blocked