2. 计算机
  3. 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

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

admin2009-05-19  57
问题 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
选项
答案B
解析 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.

In versions prior to Cisco IOS Software Release 8.3, the service subnet-zero command was used.

It should be noted that even though it was discouraged, the entire address space including subnet zero and the all-ones subnet have always been usable. The use of the all-ones subnet was explicitly allowed and the use of subnet zero is explicitly allowed since Cisco IOS Software Release 12.0. Even prior to Cisco IOS Software Release 12.0, subnet zero could be used by entering the ip subnet-zero global configuration command.

On the issue of using subnet zero and the all-ones subnet, RFC 1878 states, "This practice (of excluding all-zeros and all-ones subnets) is obsolete. Modern software will be able to utilize all definable networks." Today, the use of subnet zero and the all-ones subnet is generally accepted and most vendors support their use. However, on certain networks, particularly the ones using legacy software, the use of subnet zero and the all-ones subnet can lead to problems.
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