The subnet mask corresponding to the IPv4 address assigned to the subscriber is auto-generated in case that the IPv4 addressing authority (LUDB, RADIUS or DHCP server) does not supply it. The subnet mask is derived from the IPv4 address of the subscriber and possibly the default-gw IPv4 address and it is the smallest subnet that contains both, the IPv4 address of the subscriber and the default-gw.
For example, if the RADIUS received IPv4 address is 10.10.10.138 and the received default –gw IPv4 address is 10.10.10.170, then the subnet mask is auto-generated and set to 255.255.255.192 (/26).
138 = 10001010
170 = 10101010
192 = 11000000
In case that neither the subnet mask nor the default-gw are returned, then both would be auto-generated:
Subnet mask would be set to /31
Default-gateway which must belong to the subscriber’s subnet would be set to 10.10.10.139.
In cases where the host IPv4 address and the default-gw are directly supplied by the addressing authority but the subnet mask is missing, the subnet mask auto-generation may cause the host part of the default-gw IPv4 address to become a broadcast IPv4 address. If this is an issue, then it can be avoided by directly providing the subnet mask by the addressing authority.