127.0.0.1

One of the first things to do when testing a potentially dodgy network configuration is to ping this address. If you don't get a reply, then something is seriously FUBARd, and I would suggest reinstallation of your OS.

Also, one of a set of addresses that all internet routers will ignore, ie they will ingore all packets going to/from this address. These are 10.x.x.x, 192.16.x.x, 127.x.x.x, and here's the interesting one: 172.16-31.x.x

A fun thing to do: tell a Windows script kiddie to try hacking some complicated 127 derived address, eg: 127.56.234.191, tell them they can get lots of porn if they do it, they'll be busy all night.

If you ping 127.0.0.1, you should always get a response, with a truly minuscule latency. This is because packets sent to the loopback don't have to transit any actual network -- they go down the kernel network stack to Layer 2 and come right back. If you have a Web server running on your host, you can often test it out at http://127.0.0.1/. Pranksters may sometimes suggest that address as a place to look for warez or pr0n ....

Under no circumstances should you disable the loopback interface whilst connected to the Internet. On some systems, this may cause your kernel routing table to direct packets for 127.0.0.1 out one of your real network interfaces, where they will become unroutable "martian" packets.

On BSD systems such as Mac OS X, the interface bearing this address is usually named lo0, for "loopback interface zero". On Linux systems, it's lo -- plain lo, as Nabokov might have it.