Geeks the world over know their local host as 127.0.0.1, merely why is that specific address, of all available addresses, reserved for the local host? Read on to delve into the history of local hosts.

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The Question

SuperUser reader Roee Adler, curious virtually the default localhost IP, posed the post-obit question to the customs:

I wondered what is the origin of the decision to makelocalhost‘s IP address127.0.0.one. What is the “meaning” of127? what is the “meaning” of0.0.1?

What is the pregnant, indeed? While it’s possible to live out your unabridged geeky being not knowing the answer to those questions, we’re ready to dig in.

The Answers

Several contributors pitched in to answer Roee’s question, each one of their contributions helps shed more calorie-free on how 127.0.0.1 is the identify we all call home. John T writes:

127 is the concluding network number in a class A network with a subnet mask of255.0.0.0.127.0.0.i is the beginning assignable address in the subnet.127.0.0.0 cannot be used considering that would be the wire number. But using whatsoever other numbers for the host portion should work fine and revert to using127.0.0.i. You lot tin endeavour it yourself past pinging127.one.1.1 if you’d like. Why they waited until the last network number to implement this? I don’t think it’due south documented.

Hyperslug does some archive sleuthing by earthworks through one-time memorandums on the subject:

Earliest mention I tin find regarding 127’southward assignment as loopback is November 1986 RFC 990 authored by Reynolds and Postel:

The address aught is to be interpreted every bit meaning “this”, equally in “this network”.

For example, the address 0.0.0.37 could be interpreted as meaning host 37 on this network.

The form A network number 127 is assigned the “loopback” role, that is, a datagram sent past a higher level protocol to a network 127 address should loop back inside the host. No datagram “sent” to a network 127 accost should ever appear on any network anywhere.

Fifty-fifty as early on as September 1981 RFC 790, 0 and 127 were already reserved:

000.rrr.rrr.rrr                 Reserved                     [JBP] ... 127.rrr.rrr.rrr                 Reserved                     [JBP]

0 and 127 were the only reserved Grade A networks past 1981. 0 was used for pointing to a specific host, so that left 127 for loopback.

I know this doesn’t respond the question, but this is as far back every bit I could dig. It might accept made more sense to choose one.0.0.0 for loopback but that was already given to BBN Packet Radio Network.

While we all know and honey 127.0.0.1 as the localhost, it’s worth noting that it won’t exist the localhost forever. 127.0.0.1 is how the localhost is designated in IPv4 communications and, equally IPv6 slowly takes over, information technology will be designated by a much more intuitive number: 0:0:0:0:0:0:0:one.

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