I'm almost positive everyone on here knows the meaning of 127.0.0.1. But, why is that ALWAYS localhost? Who picked that arbitrary IP? Why was that IP picked? Why not something more simple such as 126.96.36.199? Is there some special meaning to 127.0.0.1?
locked by Chris S Feb 6 '14 at 16:10
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Jon Postel picked 127.
Before the Internet Assigned Numbers Authority took over (RFC 3232) around the time of his death (RFC 2468), he was the "czar" of Internet address and port assignments, having essentially nominated himself for the task. (RFC 349)
Back in the early 1980s, when IPv4 as we know it was first being hashed out, existing networks were given "class A" address blocks in the 32-bit address space that would go into effect in 1983 (RFC 801). Both the initial assignments and the 127 assignment you ask about, as well as the first definitions of "class A", "class B" and "class C" IP addresses, were first published in Postel's RFC 790. (Note that "classes" were superseded by CIDR in RFC 1519, now RFC 4632.)
In RFC 790, Postel defined 127 as "reserved".
Its first formal definition appears in RFC 990, where it is defined as follows:
And again in RFC 1060:
Thus, any address within 127.0.0.0/8 is to be considered loopback and be routed back to the local host.
Finally, in any IPv4 subnet, the lowest address is not usable as it represents the network route. So the first usable address in the subnet, and therefore the most commonly seen, is 127.0.0.1.
From the RFC:
Also note that 127 in binary is 01111111, the reversed and inverted value of 1 (00000001)