Garrett D'Amore wrote:
> On 5/25/2010 6:56 AM, PRDEEP KUMAR wrote:
>> Hi Experts,
>>
>> Am new to the networking.Is there any specific reason why
>> only 127.0.0.1 is used as a loop back address,why not other addresses.
>
> Historical convention? The standards say that 127/24 is reserved for
> loopback addresses.
It's actually /8. See RFC 1122 section 3.2.1.3(g), which describes an
entire Class A for loopback.
> You could use any other 127 address, I suppose (and
> I've done so), but I suspect 127.0.0.1 is so firmly entrenched in the
> minds of admins and developers that you'd probably find things that
> break if you tried a different address.
Long ago, at a company far away, we were able to use 127.x.x.x for
private communication among a collection of interconnected machines by
configuring Ethernet interfaces with addresses like 127.1.0.1/24 and by
reconfiguring lo0, which normally is configured as 127.0.0.1/8, with
127.0.0.1/24 instead. It actually worked, and allowed customers to use
the rest of the interfaces for any legal IP address, though it wasn't
what you might call "standards conformant."
I don't know of anyone who has changed lo0's address away from 127.0.0.1
... nor any reason to do so. I suspect that wasn't the original
poster's intent, though.
> Using a non-127 address would have ill effects, if it even worked. The
> reason is that the IP stack itself understands that loopback addresses
> are special, and they are given special treatment in the stack.
For what it's worth, you can configure non-127 addresses on lo0 as
aliases with no ill effect. It's how non-interface-associated
("loopback") addresses were once configured before the invention of vni,
and how they sometimes still are.
Obviously, setting lo0 itself to some non-127 address would be Very Bad,
if you could even do so.
--
James Carlson 42.703N 71.076W <[email protected]>
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