IP Subnet Zero - valid in CCNA exam?

Although this is an old post I just wanted to add a relevant note. I recently sat the CCNA 640-801 exam and they expect you to be comfortable subnetting with IP subnet zero. If they don't want you to use IP subnet-zero they will state that in the question.

Good luck to everyone.

Global,

In my past experience with the CCNA, Cisco always assumed that subnet Zero was unusable. I'm not sure if this has changed with the new CCNA exams, but I do remember that any questions that required calculations of subnets, clearly stated that you are not to use subnet zero.

So unless stated, you should most probably assume that subnet zero is not usable.

Cheers

In the old CCNA curriculum (2.x) only classful routing protocols were taught (RIP and IGRP) and use of the zero and all ones subnets were strongly discouraged. I had instructors go so far as to say they could not be used. However check this out.
www.cisco.com/en/US/tech/tk648/tk361/tec...186a0080093f18.shtml

Notice you can read it online in html or download it in pdf. Note the last couple of paragraphs as they state the zero and all ones have always been usable.

Now that the new CCNA curriculum (3.x) covers both classful and classless routing protocols (RIP v2, OSPF, and EIGRP) it would be nice if Cisco were to bring the CCNA test into reality. It has become a common practice to use the zero subnet yet still avoid using the all ones. Which reminds me of a quote I saw on a Linux FAQ site today.

"No written law has ever been more binding than unwritten custom supported by popular opinion"

Unfortunately CCNA questions are written by a team with several opinions (customs) so when it comes to sub netting because of its historic customs and evolution and apparently no written policy we can depend on they are the most controversial of questions on the CCNA and Cisco needs to develop a clear policy concerning sub netting questions because the question writers opinion of what is correct should not outweigh the facts.

I say this based on the fact that I have been told that the same team of writers that write the CCNA certification tests now writes the new CCNA curriculum module tests and course final tests. As these questions improve due to CCNA instructor feedback I would hope that would be reflected in the certification test.

I know the CCNA certification test is not 100% perfect (fair) but I would still have a tendency to answer all questions with the answers I know to be correct and hope for the best.

Very well said Mew, sometimes there is a bit of ambiguity about the exams, best to just give it your best shot.

Nice to see you back as well.

Cheers all,

I can see zero as being usable as it simply meant "the network" or the wire. However, I am not sure how all ones would work, when it means broadcast.

Also, the sentence in that article also says "However, on certain networks, particularly the ones using legacy software, the use of subnet zero and the all-ones subnet can lead to problems.".

Why tempt fate?

Yeah, when I learned networking from books etc (a long time ago) they all agreed that all zeros and all ones were 'reserved' (for the network address and broadcast address respectively) and not to be used. I get the feeling that we can get away with them these days but only because we know what we're doing (we do, don't we?) and our software and kit has become more sophisticated. So it rings true for this to be a 'legacy' issue. But anyway, as you so rightly said - why tempt fate?