IP Subnet Zero Explanations

On traditional Cisco router you maybe known and heard about ip subnet-zero command. This command will allow you to used the first subnet and last subnet of IP address. Maybe now you will never deal with this command on new Cisco code because since 12.x and now is 15.x it's turn on by default. But it's still be CCNA objective so you have read carefully question what is asking by Cisco.

I had read Cisco documentary about subnet zero and it's not just subnet zero actualy it's also known one term as all-one subnet. Just like subnetting technique known as network ID and broadcast ID. It's same with this. ip subnet zero equal to network ID and all-one subnet equal to broadcast ID. Because all-one subnet known as last subnet on CIDR.

Traditional network very strongly recommended that ip subnet zero and all-one subnet not being used on your network. the subnet zero is discussed on RFC 950

To make sure you know about IP subnet zero please see the following scenario and command on Cisco IOS router configuration.

We have IP address as you know the block subnet will be 256-192 = 64 so 0, 64, 128, 192 let's try to assign first subnet to the router.

See what happen if ip subnet-zero is disable. you will be able to use first subnet.
R1(config)#no ip subnet-zero
R1(config)#int loopback 1
R1(config-if)#ip address
Bad mask /10 for address

If we enable ip subnet-zero command then you will be able to used the first subnet and last subnet.
R1(config)#ip subnet-zero
R1(config)#int loopback 1
R1(config-if)#ip address

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