SMC Networks ADSL Barricade SMC7204BRA - Manual (Page 167)

Appendix A
170
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.