IP subnet zero命令的意义有什么？

IP subnet zero 是用來限制router不可使用與class相同的subnet.
例如: 把一個class b 的network 172.16.0.0/16 subnet細分. 其中一個細分方式係 172.16.0.0/19, 172.16.32.0/19, 172.16.64.0/19.....
而如果第一個subnet 172.16.0.0/19在沒有subnet mask的情形下 (即172.16.0.0) 與它的class network (即172.16.0.0) 相同. 這樣會引起routing上的混亂 --- 如果一個router 上有一個network 172.16.0.0/19, 而它用rip把這個network告訴它的neighbor router. neighbor router就會將所有要送去172.16.0.0 class 的packet送到這個router......
以前, RIP是很流行的routing 協議. 但它就沒有把subnet mask放入routing table. 當時亦沒有太多network admin考慮這個問題. (RFC950提到這個問題)
Cisco注意到這個問題, 所以在很早以前的IOS已經限制使用subnet zero. 以前cisco router 預設是不可使用 subnet zero (即no ip subnet zero是預設的). network admin要輸入ip subnet zero 才可使用. 從12.0開始, ip subnet zero 是預設(因為已沒有太多人使用不含subnbet mask 的routing 協議). 不過network admin還是可以用no ip subnet zero 來限制使用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。