Standard Ports And Random Ports - Watchguard Firebox X5500E Reference Manual

Transfer Protocols

Standard Ports and Random Ports

UDP and TCP use encapsulation of information contained in the application layer. The software
application procedures are specified by source ad destination port numbers. These port numbers,
together with the source and destination IP addresses, supply a unique connection on the Internet.
For example, you can have two telnet sessions from one host to a different host. Because telnet uses a
well-known service port number of 23, something must be different between these two connections.
The other port in these conditions is a port that is usually larger than 1023. The operating system on
the client side assigns this port number automatically.
Random ports can cause problems if they match a well-known service on a port higher than 1023. If
some client computer assigns a random port of 2049, no connection can be made. This type of
problem frequently occurs with the X Window and Archie services.
Most operating systems assign port numbers between 1024 and 2100 so this problem does not occur
frequently.
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