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Variable Length Subnet Mask


I think it is better to tell the difference of Subnet Mask and Variable Length Subnet Mask(VLSM) at the beginning. As I discuss in earlier article Subnet Mask is there to split one IP address into several IP’s which can use in difference LAN’s. But the question is it limit the number of accessible hosts(computers) in each LAN.  Let takes the previous example,  192.168.5.0/26 (LAN A) can only have 62 (2^6 -2) computers(hosts). But Imagine that we need to keep 100 computers in LAB A? Now I think you can understand how Subnet Mask is questioned. The answer is Variable Length Subnet Mask. I try to discuss how we use VLSM in practice in this article.
I rearrange the previous example as following.
ISP given address -> 192.168.5.0 /24
LAN A -> 100  (lets think LAN A need 100 computers).
LAN B ->50
LAN C -> 50
LAN D ->50


Let see how we solve this problem.
192.168.5.0 /24 is the given IP and we take one bit from host side of that IP. We can define two different IP’s by using that.
192.168.5.00000000 /25     and  192.168.5.10000000 /25
192.168.5.0 /25                   and   192.168.5.128 /25
We can take 192.168.5.0/25 as LAN A network address because in that IP there are 7 bits available for hosts. That mean there are 126 (2^7 -2) different hosts can be used.

In 192.168.5.128 /25 address we take another bit from available hosts bits to network side. So there are another three address can be identified. And each of them can be assigned 62 (2^6 -2) different hosts.

192.168.5.01000000 /26  -> 192.168.5.64/26 -> LAN B -> note 6 bits are available in host side.
192.168.5.10000000 /26 -> 192.168.5.128/26 -> LAN C
192.168.5.11000000 /26 -> 192.168.5.192/26 -> LAN D

with this I think I discuss all basic parts in IP addressing. Hope you may have better idea regarding this now.


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