Chapter 14. IP Addresses, Network Masks, and Subnets
Internet Security Router User
’s Manual
136
Class A networks are the Internet's largest networks, each with room for over 16 million hosts. Up to 126 of
these huge networks can exist, for a total of over 2 billion hosts. Because of their huge size, these networks
are used for WANs and by organizations at the infrastructure level of the Internet, such as your ISP.
Class B networks are smaller but still quite large, each able to hold over 65,000 hosts. There can be up to
16,384 class B networks in existence. A class B network might be appropriate for a large organization such as
a business or government agency.
Class C networks are the smallest, only able to hold 254 hosts at most, but the total possible number of class
C networks exceeds 2 million (2,097,152 to be exact). LANs connected to the Internet are usually class C
networks.
Some important notes regarding IP addresses:
„ The class can be determined easily from field1:
field1 = 1-126:
Class A
field1 = 128-191:
Class B
field1 = 192-223:
Class C
(field1 values not shown are reserved for special uses)
„ A host ID can have any value except all fields set to 0 or all fields set to 255, as those values are
reserved for special uses.
14.3 Subnet masks
Definition
mask
A mask looks like a regular IP address, but contains a pattern of
bits that tells what parts of an IP address are the network ID and
what parts are the host ID: bits set to 1 mean "this bit is part of the
network ID" and bits set to 0 mean "this bit is part of the host ID."
Subnet masks are used to define subnets (what you get after dividing a network into smaller pieces). A
subnet's network ID is created by "borrowing" one or more bits from the host ID portion of the address. The
subnet mask identifies these host ID bits.
For example, consider a class C network 192.168.1. To split this into two subnets, you would use the subnet
mask:
255.255.255.128
It's easier to see what's happening if we write this in binary:
11111111. 11111111. 11111111.10000000
As with any class C address, all of the bits in field1 through field 3 are part of the network ID, but note how the
mask specifies that the first bit in field 4 is also included. Since this extra bit has only two values (0 and 1), this
means there are two subnets. Each subnet uses the remaining 7 bits in field4 for its host IDs, which range from
0 to 127 (instead of the usual 0 to 255 for a class C address).
Similarly, to split a class C network into four subnets, the mask is:
255.255.255.192
or
11111111. 11111111. 11111111.11000000
The two extra bits in field4 can have four values (00, 01, 10, 11), so there are four subnets. Each subnet uses
the remaining six bits in field4 for its host IDs, ranging from 0 to 63.
Note
Sometimes a subnet mask does not specify any additional
network ID bits, and thus no subnets. Such a mask is called a
default subnet mask. These masks are:
Class A:
255.0.0.0
Class B:
255.255.0.0