Normally, only the root user is allowed to open ports below 1024.
That’s why, if you try running an application as a normal (non-root) user, and that application tries to open a port below 1024, you’ll get an error (access denied most likely).

If you’re running Fedora (and that would probably work on other distros too), there’s a command you can run, as root, that will allow such programs to open any of those ports, even if they’re run by a regular user.

setcap cap_net_bind_service=ep your-program-name

Example:

setcap cap_net_bind_service=ep /usr/local/bin/znc

You’ll then need to restart the program, if it was already running.

That’s it.

Kudos to stevea, a very prolific poster of FedoraForums.org (he nears 5k posts), for his answer to this question asked by another user.