ip subnet zero命令的意义

思考的牛

ip subnet zero是用来限制router不可使用与class相同的subnet.
例如:
        把一个class B 的network 172.16.0.0/16 subnet细分. 其中一个细分方式系172.16.0.0/19, 172.16.32.0/19, 172.16.64.0/19.....
如果第一个subnet 172.16.0.0/19在没有子网掩码的情形下 (即172.16.0.0) 与它的网络位(即172.16.0.0) 相同. 这样会引起routing上的混乱 ---
         如果一个router 上有一个network 172.16.0.0/19, 而它用rip把这个network告诉它的邻居. 邻居就会将所有要送去172.16.0.0 class 的packet送到这个router......
         以前, RIP是很流行的routing 协议. 但它就没有把subnet mask放入routing table. 当时亦没有太多network admin考虑这个问题. Cisco注意到这个问题, 所以在很早以前的IOS已经限制使用subnet zero. 以前cisco router 预设是不可使用 subnet zero (即no ip subnet zero是预设的). network admin要输入ip subnet zero 才可使用. 从12.0开始, ip subnet zero 是预设(因为已没有太多人使用不含subnbet mask 的routing 协议，只有RIP V1和IGRP是有类路由协议). 不过network admin还是可以用no ip subnet zero 来限制使用subnet zero.

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Two special subnets, the “zero subnet” and the “broadcast subnet,”
were reserved in years past but are now usable.

One reserved subnet, the subnetthat has all binary 0s in the subnet field, is called the zero subnet.

Subnet zero, or the zero subnet, is numerically the first subnet, but it is one of the two reserved subnet numbers in a network. You can use the zero subnet on a Cisco router if you configure the global configuration command ip subnet-zero. For the purposes of answering questions on the exam about the number of valid subnets in a network, consider the zero subnet nusable. In real life, do not use the zero subnet if you do not have to.

Example 6-21 Router A Configuration, Scenario 6-2
ip subnet-zero
no ip domain-lookup
!
interface serial0
ip address 172.16.0.1 255.255.254.0
interface serial 1
ip address 172.16.8.1 255.255.254.0
interface ethernet 0
ip address 172.16.2.1 255.255.254.0
!
router igrp 6
network 172.16.0.0

看看，这就是zero subnet :
ip address 172.16.0.1 255.255.254.0


Example 6-24 Router A Configuration, Scenario 6-3
ip subnet-zero
no ip domain-lookup
!
interface serial0
encapsulation frame-relay
interface serial 0.1
ip address 200.1.1.1 255.255.255.224
frame-relay interface-dlci 41
!
interface serial 0.2
ip address 200.1.1.161 255.255.255.224
frame-relay interface-dlci 42
!

这里：
ip address 200.1.1.1 255.255.255.224
就是zero subnet（200.1.1.0）的地址。