Understanding the network.pdf - Back to Home

M = Million
To get a better feel for CIDR, examine how you can subnet a single Class C space.
CIDR permits all the variations shown in Table 2.7.
Table 2.7. Possible Classless Variations of a Class C Address Space
Classless Mask Number of
Networks
124 255.255.255.0 1 254*
125 255.255.255.128 2 126*
126 255.255.255.192 4 62*
127 255.255.255.224 8 30*
129 255.255.255.248 32 6*
130 255.255.255.252 64 2*
*Two hosts ara lost per network for
use as network loopback and
broadcast.
Number of
Usable Hosts
Classless addressing is understood by routers—not by hosts. The hosts use their
subnet mask to determine if the destination host is local or not. When you create
supernets out of IP address spaces, use classful address subnets as beginning and
end points.
VLSM
Under classful addressing, subnetting is permitted with any classful space provided
the same mask is used to subnet the entire address space. With Variable-Length
Subnet Masks (VLSMs), this limitation is removed.
VLSM permits networks of the same class to be variably subnetted and used on the
same network. The important thing to understand about VLSM is that it is a function
of routing rather than addressing. VLSM enables routers to announce classless
variations of the same classful root address, allowing the address space to be used
more efficiently. All the subnetting options available in the classful subnetting tables
can be used with VSLM. The practical application of VLSM is discussed later in this
chapter. Calculating VLSM addresses follows the same process used to calculate
CIDR address spaces. What makes VLSM special is that it is possible to break up a
network address space into several different usable networks. Table 2.8 illustrates
such a partitioning.