IP Subnet Zero

rmiddlemass1 · ‎01-20-2015

I am working through an assignment in sub-netting where I have to use Subnet Zero. I understand this to mean that the first usable host (which is usually the sub-network address +1) is the same as the sub-network address.

My problem is that when I build my network on packet tracer to confirm it's operation the PCs object to me configuring an ip sub-network address stating that my subnet mask is invalid. However if I put in sub-network address + 1 everything is fine. But the assignment states that I must use the 0th Subnet

I take it I can configure a router in the CLI to accept IP Subnet Zero with the command "ip subnet-zero"

How can I get Packet Tracer to accept IP Subnet Zero

Any help would be great

Thank you

Ralph

Dan Lukes · ‎01-20-2015

Feedback forum is dedicated to other topics. From forum description:

Note: If your questions pertain to specific Cisco technology or solution, please post them in the proper community by leveraging the Community Directory so that folks who have expertise within those areas can engage and collaborate to it.

According your question itself, it seems you misunderstood the meaning of "Subnet Zero" term. The first useable host address is the subnet address+1 regardless the subnet is zero subnet or non-zero one. So IP address equal to address of (any) subnet is not useable and will be rejected by CLI.

The subnet zero is first (thus name 'zero' as addresses are counted from zero) subnet of a natural/implicit network block of classfull addressing scheme.

For example, 192.1.2.0 is class C network block. The 192.1.2.0/28 is the zero subnet of it, 192.1.2.16/28 is the first subnet of it and so on. The zero subnet can be used with clasfull routing scheme when enabled by 'ip subnet-zero' command only.

The first useable address of such zero subnet is 192.1.2.1, the first useable address of subnet 192.1.2.16/28 is 192.1.2.17

Rate useful advices (or mark answer as correct, if applicable). It will help others to found solutions.

rmiddlemass1 · ‎01-22-2015

Thank you for responding Dan . I thought that I had something wrong as it didn't make sense. Your explanation has clarified the situation for me.

Regards

Ralph

Dan Lukes · ‎01-22-2015

Glad to hear it helped. As I mentioned, consider rating useful advices to help others.