What’s the joke?
A frequent joke on the internet is convincing newbies to connect to 127.0.0.1, but what is this address really?
127.0.0.1 is the standard IP address used for a loopback network connection.
This means that when you try to connect to 127.0.0.1, you are looped back to your own machine.
If you connect using telnet, FTP, etc… to 127.0.0.1, you will be connected to your own machine.
So, simply put, 127.0.0.1 is you.
127.0.0.1 is also called localhost.
Actually, any IP address in the 127.x.x.x range functions in the same manner. 127.0.0.1 is just the one that is more commonly used.
The Internet RFC’s definition of 127.0.0.1
The proper use of 127.0.0.1 is defined in RFC 3330: Special-Use IPv4 Addresses:
127.0.0.0/8 – This block is assigned for use as the Internet host loopback address. A datagram sent by a higher level protocol to an address anywhere within this block should loop back inside the host. This is ordinarily implemented using only 127.0.0.1/32 for loopback, but no addresses within this block should ever appear on any network anywhere.
The IPv6 version of localhost is defined in RFC 3513: Internet Protocol Version 6 (IPv6) Addressing Architecture as ::1/128.