The other thing to do, as I have done, is just to assign your computer a made-up IP address in Network Manager. This is no problem if you have no intention of attaching your MacOS X Server box to the Internet.
This process also allows you to load all your network services up, and run all network services from your computer as if you were on a network. Yes, every DNS query will fail (since there is no outside world from your box), and you can't connect to anything outside your computer, but it's very handy for testing WebObjects and Apache and other network services, since http://localhost always works.
Same theory as giving your Mac an assigned IP address via Ethernet in TCP/IP.
This has worked fine for me, except for a couple of times when I get a weird NSException when loading the NameServer.