Asus SL6000 - Manual (Page 119)

118 ASUS VPN ADSL Router
Appendix
Appendix
A.2 Subnet masks
Mask: A mask looks like a regular IP address, but contains a pat-
tern 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 de-
fault 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 be-
cause they are used when a network is initially configured, at which
time it has no subnets.