>I have a guestion regarding VLSM
>How many subnet addresses can be summarized by 172.108.168.0/21?
>how could you calculate it?
>thanks in advance.
>
I'm not sure exactly what you are asking. Do you want to know the
total number of available host addresses in a /21? Or are you
looking for the number of subnets?
If the first, you are committing 21 bits of a 32 bit address to the
prefix, so you have the number of hosts that can fit into 32-21 = 11
bits. You compute the number of available host addresses in N bits
with the formula (2**N)-2.
If you are looking for the number of sub-prefixes, there's no
specific answer because you can set up multiple levels of subnetting.
The basic rule is:
1 /21
2 /22
4 /23
8 /24
16 /25
32 /26
64 /27
128 /28....
But you could, for example, have:
3 /23 for 500-host LANs 1 /23 divided into
24 /28 for remote office LANs
32 /30 for WAN links to remote offices
An excellent reference is http://www.isi.edu/in-notes/rfc1878.txt
And, of course, there's this excellent book by a modest guy named
Berkowitz, _Designing Addressing Architectures for Routing and
Switching_
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