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I do know that 127.0.0.1 ~ 127.255.255.254 are the loopback IP addresses for most modern operating systems and we could use these IP addresses to refer to our own computer. But isn't 127.0.0.1 enough?!?! why a wide range? why from 127.0.0.1 to 127.255.255.254?

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3 Answers 3

The 127/8 network can be used for a number of things.

1) Simulating a large number of different computers in a fast network (simply bring up more interfaces and bind services to them) without using virtual machines. This might be helpful if you wanted to have a number of different web servers running locally on port 80 for some reason.

2) Permitting more locally running services than the 64k TCP would permit (though it seems unlikely that you would hit that limit rationally)

3) Playing games with people who aren't familiar with this factoid; "Hey, you're a loser hacker, I bet you can't even hack me. Go ahead and try; I'm at 127.45.209.66"

Probably other things too.

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http://www.ietf.org/rfc/rfc1700.txt

Back in the day, IP addresses were considered as plentiful, with entire /8 and /16 blocks being given to whoever asked for them.

Mind you, even though 127/8 is reserved, the RFC states that only 127.0.0.1/32 should be used (http://www.ietf.org/rfc/rfc3330.txt).

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so.. there is no use of other IP addresses in this range except 127/32???? –  kiana Jul 5 '10 at 5:36
    
it's used for traceroute in MPLS but they could have choose another range of IP. –  radius Jul 5 '10 at 5:53
    
@Farseeker - 127/32 ? Did you mean 127.0.0.1/32 ? –  Steve Folly Jul 5 '10 at 7:37
    
@steve, yeah I did... My mistake –  Mark Henderson Jul 5 '10 at 23:17
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RFC 3330 does not state that only 127.0.0.1/32 should be used. It makes an observation that it is ordinarily implemented using only 127.0.0.1/32. The entire 127.0.0.0/8 block is still reserved for loopback. –  Juliano Jul 6 '10 at 0:24
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Back when this standard was designed, it probably made sense to have as many bits in the loopback subnet as the largest classful subnet possible, which is 24 bits. Of course we don't care about classful subnets much anymore. This is why IPv6 merely allocates one address to this purpose.

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It goes deeper than that; since 127/8 is a network address that starts with a 0-bit and has all 1-bits in the rest of the first octect, that is ALL an RFC-compliant network stack needs to look at to determine where to route traffic. The fact that this does not occur in any other address was used as an optimization, not because they thought it made sense to reserve 16 million loopback addresses (it didn't, and still doesn't). –  adaptr Dec 21 '11 at 15:14
    
Not following @TomTom - 127.0.0.1 would evaluate to 0111 1111 0000 0000 0000 0000 0000 0001 binary, which is still positive (it's the MSB that is the "sign" bit) - now, 128.0.0.1 (1000 0000 0000 0000 0000 0000 0000 0001) is negative. –  ultrasawblade Oct 15 '12 at 13:41
    
ouch, you are correct. Deleted my comment. –  TomTom Oct 15 '12 at 14:19
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