Question about IP subnet zero

TSaL

Question: In the CCENT/CCNA exams will i be able to use the first subnet in a subnetted network?

So if i have a class C address 192.168.20.0 and i need to subnet it for 60 hosts.
So using /26 I would have: 192.168.20.0 - 63
192.168.20.64 - 127
192.168.20.128 - 255

Is my first available subnet 192.168.20.0 or .64?

I have a teacher that insists that we throw out the first subnet.

In Odom books he says that you can use them if IP subnet zero is enabled.

I guess I'm worried that I'll have a question in the exam that will ask for the the first available.

Thanks for your help and i love reading the forums!

j-man

As of IOS 12.0 routers have the command ip subnet-zero in their configuration by default which means it is valid. From what I've read in the Exam-Cram CCENT, the Cisco exam should mention if the no ip subnet-zero command is in effect.

So unless told specifically, I'd use subnet zero.

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a0080093f18.shtml

From a blurb at the end of that link:

On the issue of using subnet zero and the all-ones subnet, RFC 1878 states, "This practice (of excluding all-zeros and all-ones subnets) is obsolete. Modern software will be able to utilize all definable networks." Today, the use of subnet zero and the all-ones subnet is generally accepted and most vendors support their use. However, on certain networks, particularly the ones using legacy software, the use of subnet zero and the all-ones subnet can lead to problems.

TSaL

Thank you for your response.

j-man

I should add (and I'm sorry I didn't) if you are receiving a grade from this instructor then I'd go ahead and throw out the zero subnet. If there are no grades and it's a prep for the CCENT/CCNA then I would be more concerned with what Cisco is looking for.

Welcome to the forum! Stick around and chime in now and again.