all zeroes/all ones used in host IP's...

[Henrik Olsen]

On Fri, 28 Jan 2000, Mike A. Harris wrote:
> On another mailing list I'm on there is a small discussion about
> using "0's" in IP addresses.  Nobody could categorically say
> wether or not they are allowed or not including myself, so I
> hunted down RFC 1123, and found the relevant section.
RFC 1122 :)

> Here it is:
> 
>             IP addresses are not permitted to have the value 0 or -1 for
>             any of the <Host-number>, <Network-number>, or <Subnet-
>             number> fields (except in the special cases listed above).
>             This implies that each of these fields will be at least two
>             bits long.
> 
> Now I interpreted that as meaning that none of the octets in an
> IP address could be 0 or "-1" in either the network/subnet or
> host portions of a valid host IP.  The definition of "-1" is "all
> ones" in the host or network/subnet portion.
> 
> I interpret the above as meaning that it is not legal to have a
> network like this:
> 
> 192.168.0.0/24   or 23.0.0.0/24
> 
> with hosts 192.168.0.1 through 192.168.0.254 or with hosts
> 23.0.0.1 through 23.0.0.254.
> 
> The first zero makes it illegal no?  Could someone in the know
> please clarify this as it has been bugging me for some time and
> nobody else seems to be able to say with 100% certainty what the
> proper rule is.  Also, would a network like:
> 
> 142.255.255.0/24 be illegal?
> 
> Someone has suggested that my interpretation is wrong, and if
> that is indeed so, I'd like to know the proper interpretation and
> share it with everyone.
Your interpretation is essentially correct, but has been amended by modern
style Classless Inter-Domain Routing (CIDR), where the old concept of
class A-C nets is essentially thrown away, making 192.168.0.0/16 the net
number, if you then don't subnet it 192.168.0.1 WILL be a valid host,
since the host portion is 0.1

Courses on TCP/IP still blabber on about how class A-C nets are different,
but that's essentially useless knowledge nowadays, used only by lazy or
ignorant teachers because it's easy to grade on and they confuse memorised
facts with understanding.
 
> I looked through some of the kernel source and couldn't find any
> special handling of such addresses.
Modern routing code has no knowledge of the different classes (apart from 
multicasting), and is entierly using the netmask.

> 
> Thanks very much in advance.
> Take care!
> TTYL
> 
> --
> Mike A. Harris                                     Linux advocate     
> Computer Consultant                                  GNU advocate  
> Capslock Consulting                          Open Source advocate
-- 
Henrik Olsen,  Dawn Solutions I/S       URL=http://www.iaeste.dk/~henrik/
  Blessed be the pessimist, for he hath taken backups.



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