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

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 an understatement: a server shouldn't use a well-known or registered port number to send responses back to clients. The reason is that it is possible for a particular device to have both client and server software of the same protocol running on the same machine. If a server received an HTTP request on port 80 of its machine and sent the reply back to port 80 on the client machine, it would be sending the reply to the client machine's HTTP server process (if present) and not the client process that sent the initial request.

To know where to send the reply, the server must know the port number the client is using. This is supplied by the client as the Source Port in the request, and then used by the server as the destination port to send the reply. Client processes don't use well-known or registered ports. Instead, each client process is assigned a temporary port number for its use. This is commonly called an ephemeral port number.