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!
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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.
> --
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> |
> - Lazarus Long | James Fidell
>
>
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