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I wondered what is the origin of the decision to make localhost's IP address 127.0.0.1. What is the "meaning" of 127? what is the "meaning" of 0.0.1?

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I'm curious about this too. The IPv6 loopback is 0:0:0:0:0:0:0:1, which makes sense to me. –  hyperslug Aug 29 '09 at 6:11
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Yes, IPv6's localhost does not raise too many questions :) –  Roee Adler Aug 29 '09 at 6:54
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I haven't seen a "There's no place like 0:0:0:0:0:0:0:1" door mat yet! –  William Hilsum Sep 4 '09 at 23:10
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@Wil Compress it to "there's no place like ::1" and you'll get a bit more business. –  new123456 May 4 '11 at 12:08
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@WilliamHilsum expand it to "There's no place like 0000:0000:0000:0000:0000:0000:0000:0001" and charge more. –  Nick T Apr 20 '13 at 6:44
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6 Answers

up vote 72 down vote accepted

127 is the last network number in a class A network with a subnet mask of 255.0.0.0. 127.0.0.1 is the first assignable address in the subnet. 127.0.0.0 cannot be used because that would be the wire number. But using any other numbers for the host portion should work fine and revert to using 127.0.0.1. You can try it yourself by pinging 127.1.1.1 if you'd like. Why they waited until the last network number to implement this? I don't think it's documented.

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The only reasons I would see them arbitrarily choosing 127 is that it's an easy number to remember (01111111), and perhaps they were allowing 16 million host addresses for being able to communicate with itself and itself only (like how some programs and Windows components use ports nowadays). The RFCs only really mentioned that it's standard practice for 127.0.0.1/32 to be used for loopback. It's quite nebulous as to what they intended for the rest of the block aside from it looping back to the host and never hitting the network, hence my above speculation. –  RoyalKnight Aug 29 '09 at 23:27
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Having all the block addresses reverting to 127.0.0.1 is implementation dependent and might perhaps just be a Linux stack peculiarity. Solaris at least requires an interface to be bound to the destination address for your ping test to succeed. –  jlliagre Jan 20 '11 at 1:22
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Earliest mention I can find regarding 127's assignment as loopback is November 1986 RFC 990 authored by Reynolds and Postel:

The address zero is to be interpreted as meaning "this", as in "this network".

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

...

The class A network number 127 is assigned the "loopback" function, that is, a datagram sent by a higher level protocol to a network 127 address should loop back inside the host. No datagram "sent" to a network 127 address should ever appear on any network anywhere.

Even as early 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 Class A networks by 1981. 0 was used for pointing to a specific host, so that left 127 for loopback.

I know this doesn't answer the question, but this is as far back as I could dig. It might have made more sense to choose 1.0.0.0 for loopback but that was already given to BBN Packet Radio Network.

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The designers of the Internet really knew how the hardware worked, and they designed with low level implementation in mind.

The values 0, 127 and 255 are special in 8 bit assembly and machine language programming because there are "tricks" you can use to test for these values and branch to different code using smaller instructions that execute faster than for other integers. 127 is the highest signed 8 bit integer, so incrementing it by 1 will cause a signed overflow. Similarly, incrementing 255 will cause unsigned overflow. Merely loading the value 0 into a register will usually set a zero flag on the chip. Imagine the networking program looks like this in pseudocode:

if (value == 0) doLocal();
if (value == 127) doLoopback();
if (value == 255) doNetwork();

Although it depends on the chip, in those days most chips could encode these tests with 2 words, 3 words and 3 words respectively (total 8 words) and further those particular tests were all likely to execute in 1 clock cycle each. Using any other value would probably require 4 words each (total 12 words), a 50% increase in code size and likely a 50% increase in execution time as well.

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Note that I used "words" because old machines back then had unusual word sizes, but they often still used 8 bit arithmetic. For an 8088, I would have said "bytes." –  Joseph Bui Apr 15 '11 at 15:39
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If you think about what a localhost or loopback IP address means, you realize that you never want to see that address, or the network that that address belongs to, outside of a host. (Inside of a host, it's too dark to see it. Apologies to Mark Twain.)

So, someone had to pick an IP network to represent this localhost address. I don't recall who first chose it, but it's specified in the IETF Request for Comments that is periodically issued as "Host Requirements".

It was done so long ago, that the idea of "wasting" an entire class A address didn't enter anyone's mind at the time.

The utility of localhost is that you can talk to yourself using a hard-coded IP address. It was used long before there was the Domain Name System. You could actually use any of the 127.x.x.x valid addresses, but no one ever does. You can't sneak and use 127 as a real network because the "Router Requirements" RFC disallows ever routing that network on any internet.

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First, the whole 127.x.x.x range points to your localhost.
127 in binary is "01111111". "11111111" = 255 and 0 are reserved, so the choice is obvious :)

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How come it's obvious again? –  Roee Adler Aug 30 '09 at 4:42
    
I believe they just chose something simple. Again, all "1"s is reserved, so one zero was added :) –  kolypto Aug 30 '09 at 10:15
    
so why not 10111111 ? :) or 11011111 or 11101111 or 11110111 or 11111011 or 11111101 or something like it ? –  Mushex Antaranian Oct 8 '09 at 7:40
    
Nope. Try again. –  azure_ardee Jun 11 '13 at 10:22
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Because when those standards were created, computers were slow and usually limited to 8 bit registers. Comparing numbers to numbers was very slow, expecially if those numbers had to be fetched from the back then REALLY slow memory. Registers, that is "CPU on board" storage were much faster.

Moreover those old computers had special, faster instructions to detect "equal to zero", "different than zero", "negative / positive integer" (where the sign was... guess what, the leftmost bit, now see a connection with 127, that is the number with all binary "1" except the leftmost = sign?).

Therefore those were special numbers, because they allowed programming trickery to save a lot of CPU cycles on frequently done operations.

You'd never see a "IF CallerIP = "0" but a "IF NotZero(CallerIP)" instruction.

You could go check ancient assembly instructions like "BEQ, BNE" (6502 CPUs) for longer explanations. Also check this page out.

In the end:

0, 255 and 127 could all be checked for with an one, fastest, instruction. Even more high level languages like C have "shortuct" comparison functions that the compiler may internally optimize into a one instruction.

Years 70 and 80 programmers really produced magnificent architectures with super-scarce resources, behind standards like IP numbering there's a lot of thought and genius.

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