Subnet 0 allowed or not for CCNA?

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I'm trying to find out if using subnet 0 is permitted or not for the CCNA exam? There seems to be a bit of confusion on this and my course materials say 'make sure you check whether subnet 0 is allowed or not before taking the exam'

I'm having problems finding whether it is allowed or not on the Cisco website. Has anyone taken the exam recently and know whether it is or isn't allowed?

 

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I would suppose that it will depend on the question.

I'm assuming that the questions will specify weather classfull or classless IP addressing is being used, and therefore you will know if subnet zero is allowed to be used.

 

But if I got a question giving a CIDR address asking how many subnets/hosts are available to use I'd need to know if using the 0 subnet (also the network address) is permissable or not.

There's quite an interesting debate about it here:

http://www.certcities.com/forums/forum_posts.asp?tid=2260&pn=1

I'm not due to sit the exam until the end of this month by which time I'll definitely want to be 100% sure whther the 0 subnet is a useable subnet or not.

How would I know from the exam question whether it is OK to use it or not from the address in question being classful/classless?

 

This isn't a question as to whether subnet 0 is a part of the CCNA objectives as classless routing is very much a part of the CCNA objectives. The statement is most likely directed to whether or not the routers in the network will accomodate the usage of the 0 subnet as not all routers support this.

 

I found this after a simple google search.

One of the posters who has sat the exam says that Cisco expect you to be comfortable with the use of subnet zero, but if they don't want you to answer the question using it then they say so on the question.

NB: the link was submitted in december 2004, so I don't know if the exam has changed since then.



EDIT: I just read a bit more, and now I'm more confused than before!


I think I'll just have to say: 'I don't know!'

 

That is a blinding website!

Bookmarked that one for thorough perusing- I wish the colour scheme was a bit more work friendly though so I could read it at my desk hehe:

http://www.firewall.cx/

On topic, I'm going to assume that the question will state whether using subnet 0 is permissable or not.

 

When I took my CCNA test the advice from the Cisco Press books was to know subnet 0 because it is used in cidr address schemes. However, the books advised that before you actually put it into practice to make sure that your routers supported it because many older Cisco routers did not support it. I answered all the subnetting questions on my exam under the assumption that subnet 0 was valid and I missed no questions on subnetting.

Subnet 0 didn't used to be a valid subnet in the CCNA because they didn't include protocols like OSPF that use it, but now that these protocols are a part of the CCNA subnet 0 is a valid part of the exams.

 

One more thing here. You have to know, per protocol, if subnet 0 is valid or not. It's not in a protocol such as RIP, but it is in RIP2, because RIP doesn't support classless routing, but RIP2 does as it supports classless routing schemes.

This is probably where the confusion on this subject comes from.

 

I've found an official response on the CCNA prep centre forum to this question:

That's the official response as of May 2005