ip subnet-zero

You've been assigned a single Class C address. From this, you need 8 subnets, and
your subnet mask is 255.255.255.224. Which one of the following configuration
commands would you have to use before you begin?
A. Router(config)# ip classless
B. Router(config)# ip subnet-zero
C. Router(config)# ip version 6
D. Router(config)# no ip classful
E. Router(config)# ip unnumbered
F. Router(config)# ip all-nets
Answer: B
Explanation:To get 8 subnets from a class C address, and a mask of 255.255.255.224
use the reserved subnet space. To do this, you need the command 'ip subnet-zero.'
This will allow the router to use the very first subnet, which is normally reserved
and unused as the network address.Prior to Cisco IOS® Software Release 12.0,
Cisco routers, by default, did not allow an IP address belonging to subnet zero to be
configured on an interface. However, if a network engineer working with a Cisco
IOS software release older than 12.0 finds it safe to use subnet zero, the ip
subnet-zero command in the global configuration mode can be used to overcome
this restriction. As of Cisco IOS Software Release 12.0, Cisco routers now have ip
subnet-zero enabled by default, but if the network engineer feels that it is unsafe to
use subnet zero, the no ip subnet-zero command can be used to restrict the use of
subnet zero addresses.