D-Link DSL-524T User Manual page 102

Subnet Mask
In the absence of subnetworks, standard TCP/IP addressing may be used by specifying subnet masks
as shown below.
Subnet mask settings other than those listed above add significance to the interpretation of bits in the
IP address. The bits of the subnet mask correspond directly to the bits of the IP address. Any bit and a
subnet mask that is to correspond to a net ID bit in the IP address must be set to 1.
DSL-524T ADSL Router Manual
IP Class
Class A
Class B
Class C
102
Subnet Mask
255.0.0.0
255.255.0.0
255.255.255.0