I’m assuming this is OS X. (I don’t know OS X but it’s based on FreeBSD so this command should work.)
If you have router such as Linksys and are using DHCP, an IP other than 127.0.0.1(localhost) is already set. Open a command line prompt window (Terminal Window, I believe) and type ifconfig.
Should get something like this (Note: edited output):
eth0 Link encap:Ethernet HWaddr 00:80:AD:72:E8:9A
inet addr:192.168.1.2 Bcast:192.168.1.255 Mask:255.255.255.0
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
Use the number after inet addr: from the eth0 section.
http://whatever-the-number-is
As I said I don’t really know Mac’s so there maybe a different command that does the same thing.
HTH