How do you decide what port to use?

This is a little subjective, as there are no rules so to speak. Every time I create a server, I think to myself, "What is the best port to use?" I guess an answer is "Any, as long as the user can change it." So, how does everyone else decide how to choose the default port? Personally, I like to use something like 8000-something if it's HTTP related, and I've noticed this is a pretty common trend. But what if 8000 is already in use? Use 8001? It seems a little ad-hoc, and I suppose it is.

Clearly I'm not the first to have asked this question; IANA maintain a port numbers list... Which leads me on to the unassigned range (48620-49150). I guess we should really be using these, but why don't more programmers do so? How do you decide which to use; if everyone started at #1, then we'd all be using 48620.

I think you've pretty much answered your question as much as is possible; there isn't really a strict rule you can follow here beyond what you've said. But generally:

• Look at the IANA list and pick a port that's not in use.
• Pick a port number that is easy to remember.
• Don't fix the port number in your code. Some other product may have picked the same port as you and you never know when you'll have to co-exist on a server, so put the port number in a configuration file somewhere so it can be changed without a recompile if necessary. The ability to change port number can also be helpful for getting through firewalls without having to get them reconfigured. (You can always default to your chosen value if the configuration file doesn't exist.)
• There is an argument that you don't want to pick something too high as you may conflict with the range used for ephemeral ports. It's not that likely that you'll get hit by this, but it's a hard problem to debug when it happens.

(And if you want a tip for picking memorable port numbers, I once worked with someone who remembered port numbers based around the telephone extensions of his co-workers.)

Some easy to remember and appropriately nerdy unassigned (per IANA) ports:

27182 (e)

31415 (pi)

60221 (avagadro's)

During testing... always port #666 ;)

How about:

defaultPort = (new Random()).Next(48620, 49150);

You answered your own question? Pick any unassigned port and allow the user to change it.

I prefer this way: (python code following)

#!/usr/bin/env python3
import random as R
r = R.SystemRandom()
print([r.randrange(1024, 65535) for x in range(4)])

And then I pick the number which I like the most. Or course, change the range if you have some stricter limits of what are acceptable numbers.

After a quick Google search to make sure it's clear, I generally just choose a number of personal significance.

I'd suggest never use a port that is a big number like 5 digits, as it might hit some other operation system processes and assigns the Ephemeral ports. You would start to get 'Already in use errors' due to its limitations.