Zero and Broacast Subnet issues

Unanswered Question
May 16th, 2007


I am studying for CCNA. I have 2 questions about IP subnetting and broadcast subnet.

One, if you use the /25 mask for an address and it gives you 2 subnets according to the rule of # of bits ^2 -2 you would have 2 unusable subnets. My subnets calculators do not seem to indicate they are unusable. OK, so some say using the zero subnet is fine in real world. so that would leave me with one in a real world scenario. How's this dealt with on the CCNA test?

Second question is what the heck is a broadcast subnet? I keep reading that it is when all the subnet bits are 1s. But that doesn't tell me what it does or what is being broadcast. What is it and can I use it in real-world scenarios for hosts and how is that treated on CCNA test?

Thank you!

Marc in WA

I have this problem too.
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marcsharc Wed, 05/16/2007 - 12:51

Actually, in my Cisco Press CCNA Exam Prep books there is a chart that shows that with 1 subnet bit you get 0 subnets or hosts (my math above was wrrrrong!) So how does the /25 work?

Marc in WA

Rudy_Rawlins Wed, 05/16/2007 - 15:17

First off Marc if you would control your anxiety and study your material you would be able to answer your questions.

Also asking about what is given in an exam is a violation of Cisco rules and is unethical.

marcsharc Wed, 05/16/2007 - 15:22

Wow, I didn't realize my anxiety was so apparent. I didn't mean to ask what was on the test regarding these questions, I'm really just trying to understand. I hear conflicting information about what is the proper way to use the zero and broadcast subnets.

Thanks for nothing.

dgoodridge Thu, 05/17/2007 - 03:34

Hi Marc,

Hopefully I can help with your queries but I'm not entirely sure I know what you're asking.

Firstly a /25 will give you two useable subnets when subnetting a standard CIDR block with a 24 bit mask. These are useable in the real world and I wouldn't expect any exam (Cisco or otherwise) to disagree with that.

A broadcast subnet is exactly as you stated; an address where the network portion of the address is all '1's, this again is perfectly usable as long as the host portion of the address is something other than all '0's or all '1's. This type of subnet does not broadcast anything special and shouldn't be confused with a broadcast address within a subnet.

The confusion between different calculators can arise if they assume that the 'ip subnet zero' command has or hasn't been issued.

If my memory serves me correctly, all IOS later 12.2 have the above command as default. With the command enabled you are able to have a greater number of useable subnets.

Does that help answer your questions at all?

marcsharc Thu, 05/17/2007 - 08:21

Hello Mr. Goodridge,

Thank you for replying. Actually, even though I was somewhat insulted by the insinuation that I was trying to cheat, Rudolph's answer above was right in that one answer was available in my book and I found another in a video on demand here. What I learned was that some time ago, before VLSM-aware routing protocols were common, routers had problems routing with some addresses in the zero and broadcast subnets and that is why they are "discouraged." So now my question would be, does that mean if I use RIPv1 or IGRP will they still have problems routing when those subnets are used for hosts or is there some other feature in the newer IOS that will compensate? Is that the "ip subnet zero" command you mentioned? Does that cover using the broadcast subnet too?

Another thing I'm going to tell Cisco is that if they still want to discourage the use of the zero and broadcast subnets for hosts (especially for CCNA students) then they should really change their subnet game(s) here to reflect that. BTW, the two subnet games here are great!

Would it be more accurate to say that an address with a /25 mask would be a classless IP or is CIDR more accurate or will both work?

I will continue to search for answers on my own, so don't feel obligated to answer all this. It is good to be able to chat with other people about these subjects so hopefully this will help someone else understand as well.

Thank you for your answers, Mr. Goodridge.

dgoodridge Fri, 05/18/2007 - 03:26

Hi Marc,

Lots of questions there, haven't got much time so I'll bullet the answers:

- I believe the use of RIPv1 or IGRP with the IP Subnet zero command may have unpredicatable results and should be avoided. I doubt that any any updates to the IOS would have changed the protocols behaviour surrounding this (or compensated for it).

- To better explain the ip subnet zero command take a look at this page:

- Can't comment on the games, I haven't played them! :)

- Any address and mask combination that differs from the internet standards (/8, /16 ,/24 /32) can be considered classless.

- CIDR relates to blocks of class C addresses only (/24) and CIDR questions usually relate to ranges that have been given to a company by their ISP.

Hope that helps,



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