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http://www.howtogeek.com/126304/why-is-the-localhost-ip-127.0.0.1/
Why Is the Localhost IP 127.0.0.1?
Geeks the world over know their local host as 127.0.0.1, but why
is that specif ic address, of all
available addresses, reserved for t he local host? Read on to
delve into the history of local hosts.
Image by GMPhoenix; available as wallpaperhere.
Todays Question & Answer session comes to us courtesy of
SuperUsera subdivision of Stack
Exchange, a community-drive grouping of Q&A web sites.
The Question
SuperUser reader Roee Adler, curious about the default localhost
IP, posed the f ollowing question
to the community:
I wondered what is the origin of the decision to make localhost
s IP address
127.0.0.1. What is the meaning of127? what is the meaning
of0.0.1?
What is the meaning, indeed? While it s possible to live out
your ent ire geeky existence not
knowing the answer to those questions, were ready to dig in.
The Answers
Several contributors pitched in to answer Roees question, each
one o f their contributions helps
http://www.howtogeek.com/126304/why-is-the-localhost-ip-127.0.0.1/http://www.howtogeek.com/126304/why-is-the-localhost-ip-127.0.0.1/http://www.howtogeek.com/126304/why-is-the-localhost-ip-127.0.0.1/http://gmphoenix.deviantart.com/art/Geeky-wallpaper-185468657http://www.howtogeek.com/126304/why-is-the-localhost-ip-127.0.0.1/
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shed more light on how 127.0.0.1 is the place we all call home.
John T writes:
127 is the last network number in a class A network with a
subnet mask of255.0.0.0
. 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 using127.0.0.1. You can
try it yourself by
pinging127.1.1.1 if youd like. Why they waited until the last
network number to
implement this? I dont think its documented.
Hyperslug does some archive sleuthing by digging through old
memorandums on the subject :
Earliest mention I can find regarding 127s assignment as
loopback is November
1986RFC 990authored 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 backinside 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 doesnt 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.
http://tools.ietf.org/html/rfc790#page-4http://tools.ietf.org/html/rfc990#page-6
