IP Subnet Zero

fommy

Hello everyone, im going through some sample tests on the CCENT exam, all going well so far.

Just stuck on what IP Subet Zero means and how this can make a difference to how many subnets and hosts you can have on the network?

Any simple answer would be great. Thanks,

Matt.

dynamik

Basically, older routers couldn't use subnets that had binary values of all 0s or all 1s. This was resolved with RFC 1878 in 1995, and that is no longer a limitation on any modern equipment.

If you have to use it for compatibility purposes, you will need to subtract two from your total number of subnets to get your usable number of subnets (just like you do with hosts). Otherwise, you can use every subnet.

pooker

You cant use your first and last subnet, I very much encourage you to learn this for your tests.

Webmaster

Check out the Subnetting section in our CCNA FAQ for some more dicussion on subnet zero.

@Pooker: actually you 'can' use the first and last subnet when ip subnet zero is enabled, which is the case by default. As dynamik posted, it's no longer a limitation unless you are using old and incompatible equipment (which is usually not the case on the exam ).

pooker

sorry thats what I meant, but two people in my class have taken it so far and just trying to offer some friendly advice about knowing it.