Black Box LR1530A-EU-R3 - Manual (Page 38)

Applications
32
The size of the subnet mask. defines the subnet
mask by using the specified number to reserve a
series of contiguous bit locations from the start of
the entire IP address. These reserved bit locations
are then used as the network portion of the IP
address.
For example, with a class C IP address, a subnet
mask size of 26 will mask the 24 network address
bits plus 2 host bits for the subnet address,
resulting in 4 subnet addresses being created. (Note
that depending on whether or not nonstandard
subnets are allowed, not all of these addresses may
be valid; see the sections on defining masks).
The configuration options described here are only for initial set up and
configuration purposes. For more information on all of the configuration
parameters available please refer to the router PPP Menus Reference
Manual file on the accompanying CD-ROM.