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Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Transport Layer Protocols
           9  Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)

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TCP and UDP Overview and Role In TCP/IP
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TCP/IP Transport Layer Protocol (TCP and UDP) Addressing: Ports and Sockets

Internet Protocol (IP) addresses are the universally-used main form of addressing on a TCP/IP network. These network-layer addresses uniquely identify each network interface, and as such, serve as the mechanism by which data is routed to the correct network on the internetwork, and then the correct device on that network. What some people don't realize, however, is that there is an additional level of addressing that occurs at the transport layer in TCP/IP, above that of the IP address. Both of the TCP/IP transport protocols, TCP and UDP, use the concepts of ports and sockets for virtual software addressing, to enable the function of many applications simultaneously on an IP device.

In this section I describe the special mechanism used for addressing in both TCP and UDP. I begin with a discussion of TCP/IP application processes, including the client/server nature of communication, which provides a background for explaining how ports and sockets are used. I then give an overview of the concept of ports, and how they enable the multiplexing of data over an IP address. I describe the way that port numbers are categorized in ranges, and assigned to server processes for common applications. I explain the concept of ephemeral port numbers used for clients. I then discuss sockets and their use for connection identification, including the means by which multiple devices can talk to a single port on another device. I then provide a summary table of the most common well-known and registered port numbers.

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