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Appendix A
176
Subnet masks
Definition:
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 means "this bit is part of the network ID"
- bits set to 0 means "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 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.