Internet Core Protocols: The Definitive Guide

Well-known ports

Most server-based IP applications use what are referred to as " well-known” port numbers. For example, a TFTP server will listen on UDP port 69 by default, which is the well-known port number for TFTP servers. This way, any TFTP client that needs to connect to any TFTP server can use the default destination of UDP port 69. Otherwise, the client would have to specify the port number of the server that it wanted to connect with (you’ve seen this in some URLs that use http://www.somehost.com:8080/ or the like; 8080 is the port number of the HTTP server on www.somehost.com).

Most application servers allow you to use any port number you want. However, if you run your servers on non-standard ports, then you would have to tell every user that the server was not accessible on the default port. This would be a hard-to-manage implementation at best. By sticking with the defaults, all users can connect to your server using the default port number, which is likely to cause the least amount of trouble.

Tip

Some network administrators purposefully run application servers on nonstandard ports, hoping to add an extra layer of security to their network. However, it is my opinion that security through obscurity is no security at all and that this method should not be relied upon by itself.

Historically, only servers have been allowed to run on ports below 1024, as these ports could be used only by privileged accounts. By limiting access to these port numbers, it was more difficult for ...