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Whenever a client connects to a service on
a server, a source and destination port must be specified. TCP and UDP
segments contain fields for source and destination ports.
Destination ports, or ports for services, are normally defined using
the well-known ports. Source ports set by the client are
determined dynamically.In general, a client determines the
source port by randomly assigning a number above 1023. For instance, a
client attempting to communicate with a web server uses TCP and
assigns the destination port as 80 and the source port as 1045. When
the packet arrives at the server, it passes up to the transport layer
and eventually to the HTTP service which operates at port 80. The HTTP
server responds to the clients request with a segment which uses port
80 as the source and 1045 as the destination. In this way, clients and
servers use ports to distinguish what process the segment is
associated with.
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