movistar RTA01N User Manual page 202

User's Guide
Note
255.255.255.192 or 11111111. 11111111.
11111111.11000000
The two extra bits in field4 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 1 to 62.
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.
IP Addresses, Network Masks, and Subnets
202