Re: LAN IP addresses

On Mon, 08 Sep 2008, in the Usenet newsgroup comp.os.linux.networking, in
article <ga2ub7$1vpv$1@xxxxxxxxxxxxxxxxxx>, Pascal Hambourg wrote:

Moe Trin a écrit :

None the less with the advent of CIDR, the meaning of the "network"
and "broadcast" address can not be known to outsiders,

Wasn't this true already with subnetting, even before CIDR ?

To a lesser extent, yes. But with CIDR, even the slender clues of
"Class" are missing. Is 41.190.68.0 a host, or a network? Is
24.38.143.255 a host or a broadcast? Without using a whois query,
you can't tell (neither are hosts).

But then, a lot of people misinterpret RFC1122 Section 3.2.1.3 on
page 31 (third paragraph below (g)) as prohibiting a zero (or -1
which they don't understand) in any host part.

Hey, I interpret this paragraph this way too ! :-o
I quote it for completeness :

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).

To me it clearly means that the host number part of a unicast host
address cannot be 0. What I am reading wrong ?

What is the "host number part"? Most people wrongly assume that this
refers to the right-most octet only, and thus that x.y.z.0 and
x.y.z.255 can not (except in the special cases noted) be a host. If
I have 198.18.56.0 with a mask of 255.255.252.0, can I have a host
numbered 198.18.56.255 or 198.18.57.0? Of course I can, because
the "network" address is 198.18.56.0, and the broadcast address is
198.18.59.255, and NO OTHER ADDRESS IN THAT RANGE IS SPECIAL.

Oh, and you may also want to notice the "<Subnet-number>" mentioned
above (also mentioned in RFC1812 and the various 'Assigned Numbers"
RFCs like RFC1700), and then look at RFC3330. It was common with some
operating systems that the lowest and highest _subnet_ were considered
unusable.

Old guy
.

Relevant Pages

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