SMC Networks ADSL Barricade SMC7204BRA - Manual (Page 168)

Subnet masks
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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.