Subnetting question

krjaykrjay Member Posts: 290
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?
2014 Certification Goals: 70-410 [ ] CCNA:S [ ] Linux+ [ ]

Comments

  • cisco_troopercisco_trooper Too many Member Posts: 1,442 ■■■■□□□□□□
    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.
  • universalfrostuniversalfrost Member Posts: 247
    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.
    "Quando Omni Flunkus Moritati" (when all else fails play dead) -Red Green
  • WebmasterWebmaster Admin Posts: 10,292 Admin
    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.
  • krjaykrjay Member Posts: 290
    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.
    2014 Certification Goals: 70-410 [ ] CCNA:S [ ] Linux+ [ ]
  • DarrilDarril Member Posts: 1,588
    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#4kal#4 Registered Users Posts: 7 ■□□□□□□□□□
    Yeah agree subnetting made easy very helpful!!
  • dazl1212dazl1212 Member Posts: 377
    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
    Goals for 2013 Network+ [x] ICND1 [x] ICND2 [ ]
  • WebmasterWebmaster Admin Posts: 10,292 Admin
    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.
  • dazl1212dazl1212 Member Posts: 377
    thank you for clearing that up
    Goals for 2013 Network+ [x] ICND1 [x] ICND2 [ ]
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