This read-write IP address holds the subnet mask associated with the WAN port's IP address, expressed the same way as the address itself, identifying which bits denote the network versus the host. An administrator configures this alongside ifwanIpAddress to define the correct network boundary for that port's addressing. For example, setting this mask incorrectly relative to the port's actual network segment could cause the unit to misjudge whether a destination is local or must be routed elsewhere.