A router MUST allow its IP addresses and their address masks or prefix lengths to be statically configured and saved in non-volatile storage.
A router MAY obtain its IP addresses and their corresponding address masks dynamically as a side effect of the system initialization process (see Section 10.2.3]);
If the dynamic method is provided, the choice of method to be used in a particular router MUST be configurable.
As was described in Section [126.96.36.199], IP addresses are not permitted to have the value 0 or -1 in the <Host-number> or <Network-prefix> fields. Therefore, a router SHOULD NOT allow an IP address or address mask to be set to a value that would make any of the these fields above have the value zero or -1.
It is possible using arbitrary address masks to create situations in which routing is ambiguous (i.e., two routes with different but equally specific subnet masks match a particular destination address). This is one of the strongest arguments for the use of network prefixes, and the reason the use of discontiguous subnet masks is not permitted.
A router SHOULD make the following checks on any address mask it installs:
The masks associated with routes are also sometimes called subnet masks, this test should not be applied to them.