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The TCP/IP Guide

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

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TCP/IP Application Assignments and Server Port Number Ranges: Well-Known, Registered and Dynamic/Private Ports
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TCP/IP Sockets and Socket Pairs: Process and Connection Identification
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TCP/IP Client (Ephemeral) Ports and Client/Server Application Port Use
(Page 3 of 3)

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.


Figure 199: TCP/IP Client/Server Application Port Mechanics

This highly simplified example shows how clients and servers use port numbers for a request.reply exchange. The client is making an HTTP request and sends it to the server at HTTP’s well-known port number, 80. Its port number for this exchange is the pseudo-randomly-selected 3,022. The server sends its reply back to that port number, which it reads from the request.

 


Key Concept: In most TCP/IP client/server communications, the client uses a random ephemeral port number and sends a request to the appropriate reserved port number at the server’s IP address. The server sends its reply back to whatever port number it finds in the Source Port field of the request.



Previous Topic/Section
TCP/IP Application Assignments and Server Port Number Ranges: Well-Known, Registered and Dynamic/Private Ports
Previous Page
Pages in Current Topic/Section
12
3
Next Page
TCP/IP Sockets and Socket Pairs: Process and Connection Identification
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