Just as reference point for you, nearly everything that's been
said falls under CIDR (classless inter-domain routing. The RFCs for it are
1467, 1517, 1518, 1519, 1520, and 1817. Of those I found 1519 to be the
most informative.
On Tue, 8 Feb 2000, Jason Hirsch wrote:
> I'm going to sound very very ignorant, but, ummmmm, why? What is the
> point in restricting a network to 1, 2, 4, 8, or 16 hosts? What is wrong
> with just the simple 255.255.255.0 netmask?
>
> Sendem my way, or to the list. I'll learn.
>
> Jason
>
> ----------
> Jason Hirsch, ChemEng/Chemistry
> Make it myself? But I'm a physical organic chemist!
> Visit the Dorm Room Life may never
> http://icdweb.cc.purdue.edu/~hirsch Give us another
> [EMAIL PROTECTED] Chance to do right.
>
> On Wed, 9 Feb 2000, James Fidell wrote:
>
> > Quoting Michael J. McGillick ([EMAIL PROTECTED]):
> >
> > > I'm setting up a NAT pool on a firewall. I think I understand now that
> > > the netmask is independent of the starting address for the network it is
> > > masking. Just to make sure my understanding is correct, if my network
> > > starts at, say, 192.168.1.80, I can't very well have a netmask of anything
> > > smaller than 80, right? This would mean I was trying to set up an IP
> > > range large than 255.
> >
> > The netmask doesn't relate to the network address in this way. All the
> > netmask does is specify which part of the IP address is the network address
> > and which is the host address. Effectively this means it specifies the
> > number of hosts in the network (because you can only have as many host
> > addresses as are allowed by the number of host bits in the address).
> >
> > Given a specific size network requirement, there are only some network
> > addresses that will fit that requirement. These are anything where
> >
> > (( NOT netmask ) AND network-address ) = 0
> >
> > (NOT should be the one's complement there, just to be clear).
> >
> > This also means that given a network address of 192.168.1.80, the
> > only viable netmasks are 255.255.255.252, 255.255.255.248 and
> > 255.255.255.240, for 4-, 8- and 16-host networks respectively (I've ignored
> > the 1- and 2-host ones since they're fairly degenerate cases).
> >
> > Perhaps a few other examples would be more illuminating.
> >
> > It's possible to split any network with a network address ending .0 (let's
> > say x.y.z.0) into two equal-size 128-address networks. In this case they
> > would have network addresses of x.y.z.0 and x.y.z.128. The netmask for both
> > would be 255.255.255.128.
> >
> > If we wanted to split the same network into 4 64-address networks instead,
> > we'd have network addresses x.y.z.0, x.y.z.64, x.y.z.128 and x.y.z.192,
> > with a netmask of 255.255.255.192.
> >
> > In all cases, the netmask (as a 32-bit value) is given by:
> >
> > 2^32 - <number of addresses in the subnet>
> >
> > with the restriction that the number of addresses in the subnet must
> > itself be a power of two.
> >
> > (To really put the icing on the cake, I guess I should mention that it's
> > also possible to split a network into multiple different size subnets, too.
> > All the same rules apply for each of the subnets.)
> >
> > James.
> > --
> > "Yield to temptation -- | Consultancy: [EMAIL PROTECTED]
> > it may not pass your way again" | http://www.cloud9.co.uk/james
> > |
> > - Lazarus Long | James Fidell
> >
> >
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> > as the Subject.
> >
> >
>
>
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>
- Isaiah
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