Ports for clients

Ports for clients

10.2.3 This page will discuss source ports, which are set by clients.

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 example, a client that attempts to communicate with a Web server will use TCP and assign the destination port as 80 and the source port as 1045. When the packet arrives at the server, it moves 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 that uses port 80 as the source and 1045 as the destination. Clients and servers use ports to distinguish which process each segment is associated with.

The next page will teach students about the three categories of port numbers.