[Sussex] Multiple IPs on an ADSL line

[Mark Harrison]

Do you understand subnet masks?

If so, then you'll understand the /n notation...
... it simply means "a subnet mask of N ones followed by (32-N) zeros".

So

- /8 means a subnet mask of 8 ones, then 24 zeros, or
11111111.00000000.00000000.00000000 (binary) = 255.0.0.0 (decimal)

- /16 means a subnet mask of 16 ones, then 16 zeros, or
11111111.11111111.00000000.00000000 (binary) = 255.255.0.0

- /24 means a subnet mask of 24 ones, then 8 zeros, or
11111111.11111111.11111111.00000000 (binary) = 255.255.255.0

- /29 means a subnet mask of 29 ones, then 3 zeros, or
11111111.11111111.11111111.11111000 (binary) = 255.255.255.248

This, obviously, means that the "local" part of the IP address can range
from 000 (binary) to 111 (binary), or 0-7. Of these 8 addresses, you lose
one from the top (all the ones) and one from the bottom (all the zeros)
which are reserved for broadcast and network-type purposes, leaving you with
001, 010, 011, 100, 101, 110 - a total of 6 addresses that can be assigned
to hosts.

M.


----- Original Message ----- 
From: "Alan F" <alan at slug.greenmeads.co.uk>
To: "LUG email list for the Sussex Counties" <sussex at mailman.lug.org.uk>
Sent: Tuesday, December 21, 2004 3:32 PM
Subject: Re: [Sussex] Multiple IPs on an ADSL line


> On Tue, Dec 21, 2004 at 02:45:05PM +0000, John D. wrote:
> > Could you elaborate a little on what you mean by "a /29"? I haven't got
> > to that part in the book/mag/etc etc yet! :-P
> >
> Google for "CIDR notation", it's basically a simple way of expressing
> a number of IP addresses
>
> To find the number of IP's in a CIDR range, x being the CIDR notation
> 2^(32-x)
>
> You'll probably find that google or the CCNA's on this list will
> explain it better, though. :-)
>
> Alan
>
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