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

[Linux Lists]

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.
> 
> 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.

Let's consider the host 23.0.0.2 w/ subnet 255.255.255.0 (which is in the
network 23.0.0.0/24). From what I know about IP (which could be wrong, so 
please correct me if this is the case), we have:

- Host-number = 	0x02		(last 8 bits, as defined by subnet mask)
- Network-number = 	0x170000	(first 24 bits, as defined by the
					 subnet mask)
- Subnet-number = 	0xFFFFFF00

None of them is 0 or -1 and, thus, the IP address is valid.

I hope this clarifies your doubt. Be careful with name definitions ... ;)

Regards,
Ivan


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