The TCP/IP Model
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To build networks, those who designed them started with the idea of separating networking functions in to discrete layers. Each layer performs a specific function and is transparent to the layer above it and the layer below it.
The TCP/IP Model of networking is a different way of looking at networking. Because the model was developed to describe TCP/IP, it is the closest model of the Internet, which uses TCP/IP.
The TCP/IP model breaks down into five layers:
| Application | The Application Layer provides the user with the interface to communication. This could be your web browser, e-mail client (Outlook, Eudora or Thunderbird), or a file transfer client. The Application Layer is where your web browser, a telnet, ftp, e-mail or other client application runs. Basically, any application that rides on top of TCP and/or UDP that uses a pair of virtual network sockets and a pair of IP addresses. The Application Layer sends to, and receives data from, the Transport Layer. |
| Transport | The Transport Layer provides the means for the transport of data segments across the Internet Layer. The Transport Layer is concerned with end-to-end (host-to-host) communication. Transmission Control Protocol provides reliable, connection-oriented transport of data between two endpoints (sockets) on two computers that use Internet Protocol to communicate. User Datagram Protocol provides unreliable, connectionless transport of data between two endpoints (sockets) on two computers that use Internet Protocol to communicate. The Transport Layer sends data to the Internet layer when transmitting and sends data to the Application Layer when receiving. |
| Internet | The Internet Layer provides connectionless communication across one or more networks, a global logical addressing scheme and packetization of data. The Internet Layer is concerned with network to network communication. The Internet Layer is responsible for packetization, addressing and routing of data on the network. Internet Protocol provides the packetization, logical addressing and routing functions that forward packets from one computer to another. The Internet Layer communicates with the Transport Layer when receiving and sends data to the Network Access Layer when transmitting. |
| Network Access |
The Network Access Layer provides access to the physical network. This is your network interface card. Ethernet, FDDI, Token Ring, ATM, OC, HSSI, or even Wi-Fi are all examples of network interfaces. The purpose of a network interface is to allow your computer to access the wire, wireless or fiber optic network infrastructure and send data to other computers. The Network Access Layer transmits data on the physical network when sending and transmits data to the Internet Layer when receiving. |
All application data, whether it is an e-mail, a file, an instant message, a video or voice call, is chopped into data segments and encapsulated in Transport Layer PDU's (TCP or UDP segments). The Transport Layer PDU's are then encapsulated in Internet Layer's Internet Protocol packets. The Internet Protocol packets are then chopped into frames at the Network Access layer and transmitted across the physial media (copper wires, fiber optic cables or the air) to the next station in the network.
The OSI Model uses seven layers, and differs quite a bit from the TCP/IP model. The TCP/IP model does a better job of representing how TCP/IP works in a network, but the OSI Model is still the networking model most technical people refer to during troubleshooting or network architecture discussions.
We're going to teach you the TCP/IP model from the top down beginning with the Application Layer.
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