Manual
46
Chapter 5 - IP Addresses, Network Masks, & Subnets
ASUS GigaX 1024i+
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.”
5.2 Subnet masks
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 identies 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 eld1 through eld 3 are part
of the network ID, but note how the mask species that the rst bit in eld
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
eld4 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 Field 4 can have four values (00, 01, 10, 11), so there
are four subnets. Each subnet uses the remaining six bits in eld4 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 ini-
tially congured, at which time it has no subnets.