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。