The network administrator needs to address seven Lans. RIP version 1 is the only routing protocol in use on the network and subnet 0 is not being used. What is the maximum number of
usable IP addresses that can be supported on each LAN if the organization is using one class C address block?
The answer is E-30
My question is how come it's 30 ?
The ip-subnet zero command controls the all zero-subnet ONLY, to be able to use the all 0s subnet on a router - the all 1s subnet is used by default since a while in the IOS, i even think that it was never disallowed - As of Cisco IOS Software Release 12.0, Cisco routers now have ip subnet-zero enabled by default (even before that there was a command called "service subnet-zero").