How should one go about choosing a default TCP/IP port for a new service?

Question

When developing an app that will listen on a TCP/IP port, how should one go about selecting a default port? Assume that this app will be installed on many computers, and th

Answer 1

Well, you can reference some commonly used port numbers here and try not to use anyone else's.

If by "open to the public at large" you mean you're opening ports on your own systems, I'd have a chat with your system administrators about which ports they feel comfortable with doing that with.

Answer 2

If this is for an application that you expect to be used widely, then register a number here so no-one else uses it.

Otherwise, just pick an unused one randomly.

The problem with using one in the dynamic range is that it may not be available because it may be being used for a dynamic port number.

Answer 3

Choose a number that is not very common

Answer 4

The most comprehensive list of official IANA port numbers and non-official port numbers I know is nmap-services.

Answer 5

Choose a default port that doesn't interfere with the most common daemons and servers. Also make sure that the port number isn't listed as an attack vector for some virus -- some companies have strict policies where they block such ports no matter what. Last but not least, make sure the port number is configurable.

Answer 6

Choosing an unassigned one from the IANA list is usually sufficient, but if you are talking about a commercially-released product, you really should apply to the IANA to get one assigned to you. Note that the process of doing this is simple but slow; the last time I applied for one, it took a year.