SAR715PV ADSL Ethernet Router User’s Guide
Chapter
14
. Appendix D IP Addresses, Network Masks, and Subnets
177
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
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.
