Asus GIGAX2048 User Manual page 149

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.
GigaX Series L2 Managed Switch User Guide
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.