73
Table 3.3. Unfortunately, we have an eight-bit subnet mask (i.e.,
255.255.255.0), so we are wasting 252 of the 254 available numbers on the
subnet. One possible solution to this dilemma is to use Variable-Length
As the name suggests, with Variable-Length Subnet Masks we can have dif-
ferent subnet masks for different subnets. So, for the serial link in our exam-
ple above, 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 Table 3.4.
Therefore, this subnet mask will give us only two host IPs (2
2
­ 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 our subnet mask of 255.255.255.0 (which
we could also write as 172.16.0.0/24, due to the 24 bits in our subnet mask), we
have only 254 available host addresses. However, in our Web server imple-
mentation, 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 our 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, sometimes called supernetting. In Table 3.5, let's examine what
would happen if we backed our subnet mask off from 24 bits to 23 bits.
T A B L E 3 . 4
VLSM Example 1
Decimal
255
255
255
252
Binary
11111111
11111111
11111111
11111100