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Administration Manual ip500pbxw – English
149
Subnet masks
Definition
mask
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 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 1 to 126 hosts (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 1 to 62.
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 . 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.