BT Micro Owner’s Manual
165
organisations 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 organisation 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.
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
field3 are part of the network ID, but note how the mask
specifies that the first bit in field4 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 1 to 126 hosts (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 1 to 62.
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
Class C:255.255.255.0
These are called default because they are used when a network is
initially configured.
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