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TCP/IP Reference Model
Network communication is conceptualized as a layered model, with communication taking place between adjacent layers on an individual computer, and between parallel layers on communicating computers. The program youre running (e.g. your web browser) is at the top, at the application layer, talking to another program on another computer (e.g. a web server).
In order for your web browser client application to send a request for a web page to the web server application, it has to make library and system calls which take the information from the web browser and encapsulate it in a message suitable for transport between the two programs across the network. These messages are either transport layer TCP segments or UDP datagrams. To construct these messages, the application layer calls the transport layer to provide this service. The transport layer messages are what are sent between the web browser client and the web server. The transport layer knows how to deliver messages between a program on one computer and a program on the other end of the network. Both the OSI Model and TCP/IP Model call this layer the Transport Layer, although the OSI Model breaks this layer into several different layers functionally.
The application layer represents communication between twospecific application programs. The transport layer represents how this communication is delivered between two programs. Programs are identified by numbers called service ports. The network, IP layer represents how this communication is carried between the two end computers. Computers, or their individual network interface cards, are identified by numbers called IP addresses. The subnet layer represents how this communication is carried between each individual computer along the way. On an Ethernet network, these computer network interfaces are identified by numbers called Ethernet addresses, what you are probably familiar with as your network cards burned-in hardware MAC address.