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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.
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 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.
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