Supporting Connected Routes to Subnet Zero

IOS allows the network engineer to tell a router to either allow addresses in the zero subnet or not. The motivation has to do with some older IP routing protocols that did not support the use of the zero subnet. Basically, with the ip subnet-zero command configured, IOS allows the zero subnet with no restrictions. With the no ip subnet-zero command configured, the router rejects any ip address command that uses an address/mask combination for the zero subnet. For many of the more recent IOS versions, IOS allows the use of the zero subnet.

Note that the error message does not mention the zero subnet, instead simply stating“bad mask.”

Note that the no ip subnet-zero command affects the local router’s ip address commands, as well as the local router’s ip route commands (which define static routes). However, it does not affect the local router’s routes as learned with a routing protocol. For example, R1 could be configured with no ip subnet-zero, but still learn a route for a zero subnet using a routing protocol.