ip subnet zero enables the use of subnet zero for interface addresses and routing updates. Subnetting with a subnet address of zero is illegal and strongly discouraged (as stated in RFC 791) because of the confusion that can arise between a network and a subnet that have the same addresses. For example, if network 131.108.0.0 is subnetted as 255.255.255.0, subnet zero would be written as 131.108.0.0-which is identical to the network address.

no ip domain lookup disables DNS lookups (hostname to ip address mapping) for host names. For example, if you enter "ping http://www.yahoo.com" at the hostname prompt, the PING test will not be executed successfully because the router cannot do the ip address mapping for the FQDN. The same is true if a network administrator wants to telnet from one router to another by using the router's hostname, ie "telnet hostname2."

ip classless enables classless routing behavior, ie. CIDR, VLSM.

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Victor