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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)                 9  TCP/IP Transport Layer Protocol (TCP and UDP) Addressing: Ports and Sockets

TCP/IP Application Assignments and Server Port Number Ranges: Well-Known, Registered and Dynamic/Private Ports 1 2 3 TCP/IP Sockets and Socket Pairs: Process and Connection Identification

Port Number Use During a Client/Server Exchange

So, let's return to the matter of client/server application message exchange. Once assigned an ephemeral port number, it is used as the source port in the client's request TCP/UDP message. The server receives the request, and then generates a reply. In forming this response message, it swaps the source and destination port numbers, just as it does the source and destination IP addresses. So, the server's reply is sent from the well-known or registered port number on the server process, back to the ephemeral port number on the client machine.

Phew, confusing… quick, back to our example! J Our Web browser, with IP address 177.41.72.6 wants to send an HTTP request to a particular Web site at IP address 41.199.222.3. The HTTP request is sent using TCP, with a Destination Port number of 80 (the one reserved for HTTP servers). The Source Port number is allocated from a pool of ephemeral ports; let's say it's port 3,022. When the HTTP request arrives at the Web server it is conveyed to port 80 where the HTTP server receives it. That process generates a reply, and sends it back to 177.41.72.6, using Destination Port 3,022 and Source Port 80. The two processes can exchange information back and forth; each time the source port number and destination port number are swapped along with the source and destination IP addresses. This example is illustrated in Figure 199.

TCP/IP Application Assignments and Server Port Number Ranges: Well-Known, Registered and Dynamic/Private Ports 1 2 3 TCP/IP Sockets and Socket Pairs: Process and Connection Identification