Black Box LR1530A-EU-R3 - Manual (Page 45)

Applications
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less than or equal to 255, that is the maximum
value of each 8-bit field.
The IP address is first specified and then you will
be prompted to enter the mask size.
The mask size 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 part
of the network portion of the IP address.
For example, with a class C IP address, a subnet
size of 26 will provide 2 host bits for the subnet
address resulting in 4 possible subnets. The
addresses for two of these are all ones or all zeros
and are not valid under standard subnets, leaving
two subnets available.
Configuration: The subnet mask size entered defines the size of the subnet
mask from the start of the entire IP address.
The configuration of the sub-netted class C IP network is now
completed. Remember that each of the 2 sub-networks created may
only have 62 host IP addresses defined.
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.