No review of TCP/IP networking would be complete without a review of IP addressing. In
this section we will not explain the basics of IP addressing; rather, we will focus more on
the application of variable-length subnet masking (VLSM) and the calculation of networks
as it pertains to troubleshooting in an IP environment. If you need a more detailed discussion,
see CCNA: Cisco Certified Network Associate Study Guide, 4th ed., by Todd Lammle
(Sybex, 2004).
As internetworks grew and address space became more scarce, several methodologies were
devised to extend the address space availability. One of these methodologies was VLSM. In older
routing protocols, if you wanted to subnet a major network, you had to make all the subnets the
same size. This was because the routing protocols passed only network information and did not
include subnet mask information. Newer routing protocols pass subnet information along with
the individual routes, allowing for the use of VLSM. This enables better use of address space
because network administrators can size the subnets based on the need. For example, a point-topoint
connection has only two nodes on it, and as such only needs two host addresses. Without
VLSM, if your standard subnet mask was 255.255.255.0, a /24 subnet, then 256 “addresses”
would be used on this point-to-point connection (though 256 addresses are used, only 254 are
usable by hosts). With VLSM, this same connection could use a 255.255.255.252 mask, /30, using
only four addresses—two for the hosts, one for the subnet, and one for the broadcast address. For
reference, Table 36.5 shows various subnet mask information.
One drawback to VLSM is the complexity that it adds to the network. When there was only
one mask used in an environment, the network administrators could easily memorize the subnet
information. With VLSM, however, subnet information needs to be calculated based on the
individual situation. Miscalculation of the subnets can lead to communication problems if
machines are assigned outside a subnet boundary or on a subnet or broadcast address.