Subnetting

[Mark491]

Given the IP address of 172.156.100.100 and a mask 255.255.255.224
What is the appropriate broadcast address?

option (1) 172.156.100.127
option (2) 172.156.100.128

I thought it should be option 1 but the CCNA CD exam says option 2 is correct and I do not understand why.

:confused:

[deddard]

Hi - Welcome aboard!

You're right, they're wrong.
broadcast addresses are odd numbers (because they always include the 1 bit)
the range of IP addresses for this particular question is:

Network address: 172.156.100.96
1st host: 172.156.100.97
last host: 172.156.100.126
broadcast: 172.156.100.127

the 172.156.100.128 is the next network number up.

The cisco stuff does tend to include some errors - everyone finds them!

When are you planning to take the CCNA?

[Mark491]

Hi Grand Master Geek & thanks for that answer my instructor just confirmed that too.
This one puzzled me for quite a while last night. I'm sitting for the final exam for CCNA 1 on Sat Afternoon (Australia is 10 hours ahead) It's a great course so far with lots of theory and definitions to learn. They are asking a lot of specific questions regarding the TCP/IP Model and how it relates to the OSI model and the only way to answer is by memorising the protocol definitions / layers etc.
Have done the CCNA?
Are you a chess player ?

Regards Mark

[deddard]

No - haven't done the final CCNA yet. I'm on the last leg of Semester 4 at the Academy. I took the Intro exam (half of the CCNA) last year, and will do the ICND (the other half of the CCNA) in a few weeks. At the minute I'm working on the final case study (wide area network with frame relay, dhcp, nat etc) and then I've got half a dozen exams to do to finish the academy, then I'' do the ICND.
Don't play chess - don't have time to play anything!

Unfortunately there's a lot of memorising to do for any techie exams like this. If you can remember the basics of the TCP/IP model and what each one does, and the same with the OSI you'll crack it. Bear in mind that this stuff starts to sink in later on - semester 2 and 3 is when things start to make sense (they have to - you've got a large case study to complete in sem 3!)

[Variable]

I did my CCNA last year. I think you get the best help doing labs. Create a subnet cheat sheet with rows and columns tht correspond to each other and copy it down. When you sit for the exam write it all out. That way you don't have to do the math in your head you can look down and be right 100% of the time. Of course you have to remember the number placement, but if you write it down twenty odd times or so You will have it set in your noggin. That way if they say whats a valid host number for a /27 you look down for the number under /27 and start adding that with the scratch pad. You do that and subnetting is cake. Then you start seeing how the numbers correspond to each other and it becomes easier to see the bigger picture. Break everything down into small parts and build up.KISS



Grand Master Geek is a title you get as you post more messages. It doesn't mean he's a 2500 or whatever it is now-a-days : )

V

[Mark491]

That sounds like a great idea, Can you post a sample of the chart? I just passed my CCNA 1 final exam on Sat. But the feedback said subnetting was a weakness. They had some stuff we had not studied from the text book :eek:

[Roady]

Forgive me if I missed something here, but going back to the original question, isnt 172 (first octet of the given IP address) a class "B" network which range from 128-191. So in this case the broadcast address would be 128 as CCNA stated.

Thanks

[deddard]

Hi Roady- normally you would be right, but look at the subnet mask and you will see that the broadcast (and network address) are not class B - we are now in the realms of classless addressing with this question, so the standard A,B,C etc are meaningless.

[Roady]

ok, but how do you actually work this out using logical mathematics? Ive a basic understanding about binary and subnetting, but unsure how to work out the broadcast address.

Cheers!

[deddard]

The best advice I can give without going into lengthy explanations is to visit certiguide.com (created by IXL) which will give plenty of info - there is a section there on the Net+ exam which will be of interest.
You should also check out the TCP/IP guide, again written by IXL.

[cscoeng]

Here's a program that generates an infinite number of subnet questions in the form of practice tests. Its like a boson type test but subnetting only. Check out:

http://www.iscinc.com/SubnetTutor/

Also includes a detailed tutorial and subnet calculator.

cscoeng

[deddard]

I just checked out the link, and I'd say that anyone should be cautious about using this if the screenshot is anything to go by - the answer is wrong!

[poppy]

I would be suspicious too for a New Member with only one post to bring up a thread that was started 5 months ago and the answer seemed resolved 2 months ago. Spam maybe? Prove me wrong.