Subnetting question

krjay

I've been doing quite a bit of subnetting recently and its starting to click. But then I ran into a question with a little different format and asks a question I'm not used to.

Suppose an ISP owns the block of addresses of the
form 101.101.128/17. Suppose it wants to create four subnets from this block, with each block having the
same number of IP addresses. What are the prefixes (of form a.b.c.d/x) for the four subnets?

This is the answer it gives me:
101.101.101.64/28
101.101.101.80/28
101.101.101.96/28
101.101.101.112/28


How would you go about arriving at this answer?

cisco_trooper

The question is lame. If you have a 101.101.128.0/17 block, the four subnets you have given don't even fall within the /17. This question is stupid and I would ignore it.

If I were to break the 101.101.128.0/17 block into four equal pieces, I would create four /19s out of it:
101.101.128.0/19
101.101.160.0/19
101.101.192.0/19
101.101.224.0/19

I would double check those as I did them in my head.

universalfrost

glad that you are trying to figure out the wonderful world of subnetting. Now, when you are ready to take the Net+ exam you will maybe see a subnetting question, but more than likely only to see class questions. I only saw subnetting questions on the Server+ exam or of course my CCNA exams.

use the lammle method or google danscourses on youtube for his version. both are great and geared towards real world scenarios.

Webmaster

Actually calculating subnets, prefixes etc is beyond Network+. Just make sure you know what subnetting is, why it's done, and what numbers go with the default classful masks like a class C allowing for 254 hosts.

krjay

It was an example problem from a university course. The professor claimed the class follows a learning path that will prepare you for a network+ cert if you chose to go that route. I think I'm fine with what is expected for network+ but I just flounder when I get a problem like this. A problem that involves calculating subnets and prefixes as you call it.

Darril

krjay wrote: »

It was an example problem from a university course. The professor claimed the class follows a learning path that will prepare you for a network+ cert if you chose to go that route. I think I'm fine with what is expected for network+ but I just flounder when I get a problem like this. A problem that involves calculating subnets and prefixes as you call it.

Check out this thread. http://www.techexams.net/forums/ccna-ccent/38772-subnetting-made-easy.html

Several people spent a lot of time showing the detail that you're asking for, along with other links to get more information.

Darril Gibson
Security+ blog
Security+ Tip Of Day

kal#4

Yeah agree subnetting made easy very helpful!!

dazl1212

Stupid question.
On the subnetting made easy thread it says 5^2 is 32. I thought it was 25, is this because we are going to the 5th position in an octet I.E 16?
Just wanted to check

Webmaster

dazl1212 wrote: »

Stupid question.
On the subnetting made easy thread it says 5^2 is 32. I thought it was 25, is this because we are going to the 5th position in an octet I.E 16?
Just wanted to check

Common mistake/typo and likely meant 2^5, which is 2*2*2*2*2=32.

dazl1212

thank you for clearing that up