Is it for an internal network? If so, simply entering the IP address should be sufficient. 🙂
So if you’re accessing the library on the same PC, simply enter 127.0.0.1 or localhost.
If not, something like 192.168.1.x or 10.0.0.x should do it. Ofcourse, you should enter the IP of the system running mt-daapd. 🙂