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

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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