Variable-Length Subnet Masks

Variable-Length Subnet Masks
As the name suggests, with variable-length subnet masks (VLSMs) we can have different subnet
masks for different subnets of the same classful network. So, for the serial link in the preceding
example, we could have a subnet mask of 255.255.255.252. If we do the math and look at our
subnet in binary, we see that we have only two host bits, as shown in our first VLSM example.
Therefore, this subnet mask will give us only two valid IP addresses (22 – 2 = 2), which is
exactly what we need for our serial link.
As another example, consider what would happen if we were running out of IP numbers on
a particular subnet. Perhaps we have several web servers on network 172.16.10.0, with a subnet
mask of 255.255.255.0 (which we could also write in CIDR notation as 172.16.10.0/24, due
to there being 24 bits turned on in our subnet mask). With only eight bits left over for host identification,
we have only 254 available host addresses.
However, in our web server implementation, each URL needs its own IP number, and our
need for unique IP numbers is about to grow beyond the 254 numbers that we have available.
It is possible to support several URLs to an IP address, but for this example we’ll use only one
address per URL.
Yet again, VLSMs can provide a solution. Instead of making our subnet mask longer, as in
the previous example, we can make our subnet mask shorter. This is called route aggregation,
in general, or supernetting when dealing with a prefix length shorter than the default for the
class in question.
Decimal 255 255 255 252
Binary 11111111 11111111 11111111 11111100

Our second VLSM example demonstrates would happen if we reduced our prefix length
from 24 bits to 23 bits.

Decimal 255 255 254 0
Binary 11111111 11111111 11111110 00000000

We now have a network of 172.16.10.0 with a subnet mask of 255.255.254.0, which can
also be written as 172.16.10.0/23. Again, by doing the math (29 – 2 = 510, because we have nine
host bits), we see that we now have 510 available IP addresses instead of 254.
Let’s work through a VLSM design example, as depicted in Figure 2.2.