GigaX Series L3 Managed Switch User Guide
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 field4 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 initially configured, at which time it has no subnets.
