Subnetting Question I got wrong.

showboshowbo Posts: 73Member ■■□□□□□□□□
So here is the question:

8. You are assigned a Network ID of 145.19 and asked to configure the network to provide at least 100 useable subnets with at 500 hosts on each subnet. What is the ENDING IP address of the EIGHTH useable subnet in the network?

A: 145.19.17.254



Maybe I just can't see it, I answered 145.19.18.254 which I am wrong with my own wrong answer, the last useable would be 145.19.19.254.

But I just don't see how its 17.254, I thought the 145.19.16.0 was the 7th, not 8th?

145.19.0.0 - Not usable
145.19.2.0 - 1st useable
4.0
6.0
8.0
10.0
12.0
14.0
16.0 -8th useable
18.0 - 9th useable

Wow, before I even posted this I found why I was wrong. I counted 2.0 as unusable icon_sad.gif. Fail. icon_rolleyes.gif Since I already typed this out, I hope this helps someone else in any way!


-Oscar

Comments

  • sratakhinsratakhin Posts: 818Member
    Why did you assume that 145.19.0.0 was not usable? Did the question specifically state that ip subnet zero wasn't allowed?
  • spd3432spd3432 Posts: 224Member
    Oscar -
    I'm confused by the wording and don't know where this question came from. In my mind, the "ENDING IP ADDRESS" is different from the "LAST USABLE IP ADDRESS" in a range. The "ENDING IP ADDRESS" would be the broadcast address and for your numbers would have to end in .255. The "LAST USABLE IP ADDRESS" would be a .254 address. Your x.x.19.254 you said you used would be wrong. The x.x.17.254 would be correct if "no ip subnet-zero" was part of the configuration. If the question came from a **** site, lose the link. If it came from a book, go to the publisher's / author's websites and look to see if an errata has been published.
    ----CCNP goal----
    Route [ ] Studying
    Switch [ ] Next
    Tshoot [ ] Eventually
  • NetworkVeteranNetworkVeteran Posts: 2,338Member ■■■■■■■■□□
    showbo wrote: »
    8. You are assigned a Network ID of 145.19 and asked to configure the network to provide at least 100 useable subnets with at 500 hosts on each subnet.
    Okay. So you know 145.19.0.0 has 16 network bits (by virtue of them telling you 145.19 is the network part, or by virtue of realizing "145" means it's a class B address).

    If you need 100 subnets, you need to find the smallest X such that 2^X > 100. 2^6 = 64. 2^7 = 128. You need 7 subnet bits.

    If you need 500 hosts, you need to find the smallest Y such that (2^Y - 2) > 500. (2^8 - 2) = 254. (2^9 -2) = 510. You need 9 host bits.

    In conclusion, your answer has: 16 network bits, 7 subnet bits, and 9 host bits.

    What is the ENDING IP address of the EIGHTH useable subnet in the network?
    With 7 subnet bits, we're going to be counting by twos in the third octet.
    145.19.0.0 - 1st
    145.19.2.0 - 2nd
    145.19.4.0 - 3rd
    145.19.6.0 - 4th
    145.19.8.0 - 5th
    145.19.10.0 - 6th
    145.19.12.0 - 7th
    145.19.14.0 - 8th
    145.19.16.0 - 9th

    The last/ending IP address of the 8th useable subnet is 145.19.15.255.
  • NetworkVeteranNetworkVeteran Posts: 2,338Member ■■■■■■■■□□
    showbo wrote: »
    A: 145.19.17.254

    Showbow, since that answer is categorically wrong on multiple counts, consider using a better source for your study preparations.. maybe Odom or Lammle.

    The errata check is also a good suggestion.
  • NetworkVeteranNetworkVeteran Posts: 2,338Member ■■■■■■■■□□
    I found your source. It's riddled with errors. Here's a selected paragraph--
    IP addresses cannot be all ones or all zeros;

    Sure they can. I commonly see 0.0.0.0 and 255.255.255.255 on networks.

    therefore, in most cases the first range of addresses and the last range of addresses are unusable.
    Flawed logic. Even if 0.0.0.0 and 255.255.255.255 weren't unusable--and they certainly are used on modern networks--that wouldn't prohibit using 145.19.0.0.
    (In some special circumstances, you can use the first range of addresses, or subnet 0. Only certain manufacturers’ equipment, such as Cisco Systems, fully supports the use of subnet zero.)

    Bullhockey. Per RFC 1878 way back in 1995 (seventeen years ago), "This practice is obsolete! Modern software will be able to utilize all definable networks."
  • showboshowbo Posts: 73Member ■■□□□□□□□□
    Guys, I answered the question based on old ideals apparently. But the question was meant to be asked with the no ip subnet zero command. meaning, I couldn't use the 0.0. Just like how my Router question I posted last night, it wouldn't allow to me to use 192.168.1.1 because no ip subnet zero was in the config.

    I just randomly found this on google, and out of the 10 practice questions that was the only one I got wrong using their methods. I guess that doesn't matter...I am reading Todd and Wendells books though. I will make sure to focus more on their teachings than some random google questions icon_sad.gif.
  • NetworkVeteranNetworkVeteran Posts: 2,338Member ■■■■■■■■□□
    showbo wrote: »
    I will make sure to focus more on their teachings than some random google questions icon_sad.gif.
    Hey, mate, we just don't want you to learn the wrong things. Rock on. :)
  • iwannaknowITiwannaknowIT Posts: 111Member
    showbo wrote: »
    So here is the question:

    8. You are assigned a Network ID of 145.19 and asked to configure the network to provide at least 100 useable subnets with at 500 hosts on each subnet. What is the ENDING IP address of the EIGHTH useable subnet in the network?

    A: 145.19.17.254



    Maybe I just can't see it, I answered 145.19.18.254 which I am wrong with my own wrong answer, the last useable would be 145.19.19.254.

    But I just don't see how its 17.254, I thought the 145.19.16.0 was the 7th, not 8th?

    145.19.0.0 - Not usable
    145.19.2.0 - 1st useable
    4.0
    6.0
    8.0
    10.0
    12.0
    14.0
    16.0 -8th useable
    18.0 - 9th useable

    Wow, before I even posted this I found why I was wrong. I counted 2.0 as unusable icon_sad.gif. Fail. icon_rolleyes.gif Since I already typed this out, I hope this helps someone else in any way!


    -Oscar

    If you count from the 2.0 network(with 2.0 being the first),the 8th subnet would be 16.0 and the broadcast of that subnet would be 17.255(cannot use broadcast).So the lasy usable ip address in that subnet woud be 145.19.17.254.......
  • NetworkVeteranNetworkVeteran Posts: 2,338Member ■■■■■■■■□□
    If you count from the 2.0 network(with 2.0 being the first)
    Then you missed a number. ;)
    the broadcast of that subnet would be 17.255(cannot use broadcast).
    The question wasn't, "What is the last host address?". If it had been, then .254 would be okay. As stated, "What is the ENDING IP address?" .255 is the best answer

    (Note: As an aside, broadcast addresses are certainly useable. RIPv1, ARP, and some other protocols rely on broadcasts. Directed broadcasts can be really helpful when debugging.)
  • iwannaknowITiwannaknowIT Posts: 111Member
    Then you missed a number. ;)


    The question wasn't, "What is the last host address?". If it had been, then .254 would be okay. As stated, "What is the ENDING IP address?" .255 is the best answer

    (Note: As an aside, broadcast addresses are certainly useable. RIPv1, ARP, and some other protocols rely on broadcasts. Directed broadcasts can be really helpful when debugging.)

    This proves that Cisco Cert tests aren't tricky....it means we need if i know exactly how it works,the answer is as simple as the nose on my face...lol
    Thanks......
  • lantechlantech Posts: 329Member
    For the exams you are going to need to know when you can and can't use the zero subnet. Cisco may not explicitly state whether you can or not. They will expect you to know under what circumstances it is and isn't allowed.
    2012 Certification Goals

    CCENT: 04/16/2012
    CCNA: TBD
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