127.0.0.1

This is one of a number of non routable addresses built into the TCP/IP stack. While this address itself is used as a loopback for the purpose of troubleshooting the responsiveness of both the network card and TCP/IP, the other non routable addresses are commonly used for home networking purposed. These address are 192.168.X.X and 10.X.X.X.

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.

127.0.0.1, being the first IP address on the 127.0.0.0/8 netblock reserved for loopback use, is the address most commonly used for a loopback network interface. Every properly functioning TCP/IP network stack has one of these -- it is an address reserved for the host to use when talking to itself.

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.

One of the interesting things about the choice to use this network address was the loss of a complete A-block of addresses (a total of 16,777,215 addresses, not all of them usable). Back in the day, they never thought this Internet thing would take off, and that we'd be running out of addresses so quickly. If they had really thought about it, they could have designated something like 192.168.255.1, which would have only wasted 256 total IP addresses, 254 which were usable.

When troubleshooting your network, pinging 127.0.0.1 does check your TCP/IP stack. If it doesn't work (and you reboot to try again), you may have to reinstall the stack. You can reload the whole OS, but it's not required. In layman's terms, you're testing whether your motherboard can talk to your network card via the software.

Additional useful uses of 127.0.0.1 is to edit your LMHOSTS and HOSTS files. If you want to prevent your computer from talking to Myspace.com, for example, just put an entry that myspace.com is located at 127.0.0.1. When your kid or roommate tries to go to the malware-infested site, it ends up getting an error. The operating system checks these files when looking for a URL, and the HOSTS file is viewed first. If it's in there, the computer doesn't even bother asking a DNS server for the right IP address.