[Gllug] DHCP & DNS

[Pete Ryland]

Warning, pedantry ahead...

On Fri, Dec 07, 2001 at 08:47:28PM +0000, Ian Northeast wrote:
> Leigh Mason wrote:
> > hold on, i thought it was one of the  rfcs that stated, when using class
> > A addresses the first bit must always be off, class b must always start
> > 10xxxxxx and class c is 110xxxxx. any address that starts from 192 - 223
> > is a class c.
> > and i think the rfcs also state default subnet masks to be used for each
> > address, therefor 192.168.0.0/16 is not a default subnet, you have
> > borrowed hosts bits to create a new subnet - why bother?
> 
> There is an RFC which states this but it is out of date. There is no such
> thing as a network class any more and any network number can be used with
> any network mask (as long as you stay out of the multicast etc. ranges).
> So using 192.168/16 or 10.x.x/24 are both perfectly valid.
> 
> Many OSs (and indeed routers - Ciscos can be guilty of this) still default
> to classful routing but it can always be disabled.

Even with CIDR aside, the 192.168.0.0/16 reference was:

> From: David Damerell <damerell at chiark.greenend.org.uk>
> Date: Fri, 7 Dec 2001 09:47:21 +0000 (GMT)
>
> No! Not 192.168.0.0/24! Pick a _random_ subnet of 192.168.0.0/16 or
> the reserved class Bs specified in RFC 1918. Does no-one actually read
> any standards anymore?

So the 192.168.0.0/16 was used (implicitly) to refer to a block of 254 class
C subnets, and not actually referring to a /16 subnet at all.

As an aside, I think it's funny how no-one ever remembers the ip range for
the class B addresses reserved for private networks.  "It's, er, 172 dot
something something..." :)

Pete
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