Asus SL1200 - Manual (Page 164)

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Chapter 13 - IP Addresses, Network Masks, and Subnets
ASUS SL1200
13.3 Subnet masks
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.
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, at which time it has no
subnets.