ATL Telecom AM30 - Manual (Page 159)

ATL Telecom User Guide
AM30
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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.