Subnet masks
Denition
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.”
Subnetmasks are used to dene 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 10.0.0. 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 eld3 are part of the
network ID, but note how the mask species that the rst bit in eld4 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 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 eld4 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 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
congured, at which time it has no subnets.
Troubleshooting
This appendix suggests solutions for problems you may encounter in installing or
using the Wireless ADSL2+ Router, and provides instructions for using several IP
utilities to diagnose problems.
Contact Customer Support if these suggestions do not resolve the problem.
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