ch03 IP Addressing.pdf - The Cisco Learning Network

22 Chapter 3: IP Addressing
Subnet Masks Values
Given the fact that subnet mask values must have all 1’s contiguous and all 0’s
contiguous, Table 3-8 shows some valid decimal numbers for subnet masks in an octet.
For a Class A network, the default subnet mask is 255.0.0.0: the first octet is
the network number and the last three octets are the host numbers. For a Class B
network, the default subnet mask is 255.255.0.0: the first two octets are the network
number and the last two octets are the host numbers. For a Class C network, the
default subnet mask is 255.255.255.0: the first three octets are the network number
and the last octet is the host numbers.
One important item to point out is that the subnet mask, in and of itself, means
nothing without the context of the IP address associated with it. For example, most
people would assume that when you see a subnet mask of 255.255.255.0, you are
dealing with a Class C address. However, remember that you can perform subnetting
on any class address. So this mask can also be used for Class A and B addresses.
Therefore, the IP address and subnet mask have a symbiotic relationship. The
following sections will show you the valid subnet mask values for Class A, B, and
C networks.
Subnet Masks for A-Class Networks
Table 3-9 shows valid subnet masks for Class A networks. In this table, the number
of networking bits is the total number of bits used in networking, including both the
network and subnet bits. This is also true with Tables 3-10 and 3-11.
Subnet Masks for B-Class Networks
Table 3-10 shows valid subnet masks for class B networks.
Subnet Masks for C-Class Networks
Table 3-11 shows valid subnet masks for class C networks.
As you can see from Tables 3-9, 3-10,
and 3-11, you can’t just choose any subnet
mask and apply it to any class of addresses:
You will need to be very
familiar with subnet masks for a given
address and the number of networks that
a subnet mask creates and the number
of host addresses for each network.
Some masks are valid for some classes, but
not valid for others. For instance, 255.255.0.0
is a valid mask for Class A and B networks,
but is an invalid mask for Class C networks.