127.0.0.1 ?

[Chewy509]

Hi Guys,

Doing some research on the origin of 127.0.0.1.

I know that 127.0.0.1/8 is the IP address assigned to the loop back device/driver as defined in RFC 1122 # 3.1.2.3 ( http://tools.ietf.org/html/rfc1122#page-29 ) and in RFC 5735 # 3 ( http://tools.ietf.org/html/rfc5735#page-3 ), and is typically mapped to 'localhost'.

However I can't find any information on why this address block was chosen for the loop back interface in IP v4?

As far as I can tell, it was just an arbitrary choice made by the designers of the IPv4 protocol? (It is the last Class A address block, may be that's it)?

Anyone have any ideas?

PS. In IPv6, the loop back IP is ::1 / 128 ? (RFC 3513 # 2.5.3 - http://tools.ietf.org/html/rfc3513#section-2.5.3 )

 

[Mercutio]

Well, 127 is halfway between 0 and 255. And the whole block is technically available for loopback needs.

 

[Howell]

Chewy, did you find the why for the 10 block and the 192 block?

I think it is little more than arbitrary.

 

[ddrueding]

How about 169.254.xxx.xxx?

 

[Mercutio]

Chewy, did you find the why for the 10 block and the 192 block?

172.16.0.0/16 never gets any love.

Also

192.168.0.0 is also defined as a /16, despite being on the wrong side of the traditional Class C boundary.

 

[Chewy509]

Chewy, did you find the why for the 10 block and the 192 block?

I think it is little more than arbitrary.

No and that's the thing. I can find out which blocks are reserved, but not why each block was chosen the way it was?

Even for private blocks allocations, it's not like they are the first block within that class (which would make sense).

Maybe for the private blocks, it was due to earlier allocations already being in place?

http://www.iana.org/assignments/ipv4-address-space/ipv4-address-space.xml

But this doesn't highlight that theory?

Maybe time to start digging through usenet?

A quick summary of reserved blocks:

0.0.0.0/8           "This" Network             RFC 1122, Section 3.2.1.3
10.0.0.0/8          Private-Use Networks       RFC 1918
127.0.0.0/8         Loopback                   RFC 1122, Section 3.2.1.3
169.254.0.0/16      Link Local                 RFC 3927
172.16.0.0/12       Private-Use Networks       RFC 1918
192.0.0.0/24        IETF Protocol Assignments  RFC 5736
192.0.2.0/24        TEST-NET-1                 RFC 5737
192.88.99.0/24      6to4 Relay Anycast         RFC 3068
192.168.0.0/16      Private-Use Networks       RFC 1918
198.18.0.0/15       Network Interconnect
                    Device Benchmark Testing   RFC 2544
198.51.100.0/24     TEST-NET-2                 RFC 5737
203.0.113.0/24      TEST-NET-3                 RFC 5737
224.0.0.0/4         Multicast                  RFC 3171
240.0.0.0/4         Reserved for Future Use    RFC 1112, Section 4
255.255.255.255/32  Limited Broadcast          RFC 919, Section 7
                                               RFC 922, Section 7

 

[ddrueding]

Sounds like an "Ask Slashdot" would do some good; they have some serious old-timers around there...

 

[Chewy509]

Some answers.
127/8 become a reserved address range with RFC790. Which also defines subnets as classes for the first time. 127/8 was marked as reserved as it was the last/highest Class A block. Formalised Loopback methodology was defined at the same time, and given this range.

10/8. Originally it was assigned to ARPANET, and with the development of NAT and Private Address Space, was reallocated as a Class A Private Address Range. (Most other Class A's had already been assigned). RFC 1918

172.16/12 and 192.168/16 were both assigned when NAT and the Private Address Space was defined. (These ranges were done, so you could define multiply Class B and Class C addresses within a single private range).

169.254/16 was only formally allocated with RFC 3927 in 2005. This was done to formalise several exsiting vendor specific methods being used to IPv4 autoconfiguration. (Blame MS for this address range).

Hope this answers my own question?

(It's all in the old and obsolete RFCs, you just need to dig through them all).

 

[Chewy509]

PS. RFC 990 is the first RFC to formaly define 127/8 for loopback.
RFC 790 defines it as a reserved address, due to ongoing development of IPv4 and the need to have a loopback interface.

 

[time]

Have you considered an academic career?

Seriously, you've already written a compiler, you're obviously a good researcher and you love minutiae. (Yes I know, the last two criteria would describe a lot of people here, but taken as a whole ...).

 

[Chewy509]

Have you considered an academic career?

Seriously, you've already written a compiler, you're obviously a good researcher and you love minutiae. (Yes I know, the last two criteria would describe a lot of people here, but taken as a whole ...).

Research and Development, yes. Academic, no. I've only been at Uni 8 weeks now, and some of the snotty/bratty 18yr olds already driven me &^%&%^*.

This question really came up, because I was stupid enough to ask in one of the lectures on networking, "I know 127.0.0.1 is the loopback IP, but why was that address chosen and not 1.0.0.0/8 or 0.0.0.0/8 like it's done in IPv6?". I was told it would be a "good research topic for your assignment". The current assignment for that class includes further self paced research items (2x research items per week, related to the current weeks lecture topic) to be included with the final report. The other research item I did for networking was IPv6. (easy, since the networking topic only brushed on IPv6 existence - it's an intro class to IT in general - not an in depth network technologies class).

 

[LiamC]

Chewy, that's an absolutely awesome snippet. Thanks.

And Merc, I never knew you could use 172...

 

[LunarMist]

Ah, so my printer is 169.254... for a reason. I printed a label for it so I can remember. :dwarf: