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TCP/IP Guide by Charles M. Kozierok

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TCP/IP Client (Ephemeral) Ports and Client/Server Application Port Use

The significance of the asymmetry between clients and servers in TCP/IP becomes evident when you examine in detail how port numbers are used. Since clients initiate application data transfers using TCP and UDP, they need to know the port number of the server process. Consequently, servers are required to use universally known port numbers. Thus, well-known and registered port numbers identify server processes. Clients that send requests use the well-known or registered port number as the destination port number.

In contrast, servers respond to clients; they do not initiate contact with them. Thus, the client doesn't need to use a reserved port number. In fact, this is really ...

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