Sobnet Zero
Old IP subnetting rules, where all 0’s subnet was reserved for the network. 0’s subnet wasn’t really used . An example of an IP address that is using a zero subnet is 10.0.1.1 with a subnet mask of 255.192.0.0. There is a 0 in third octet so for some people can be strange. However, today, this IP address is legal when it comes to subnetting. Thus, if I had an IP address of 10.5.0.0 with a 255.255.0.0 subnet mask and wanted to subnet it, I could actually get 255 valid networks out of it by using the 0 subnet.
Example:
ip subnet-zero is disabled:
R2(config)#no ip subnet-zero
R2(config)#int fa0/1
R2(config-if)#ip address 10.0.1.1 255.192.0..0
Bad mask /10 for address 10.0.1.1
If we add the subnet-zero command back:
R2(config)#ip subnet-zero
R2(config)#int fa0/1
R2(config-if)#ip address 10.0.1.1 255.255.255.0