GigaX Series L2 Managed Switch User’s Guide
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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.