Serial interface addresing

MikdillyMikdilly Member Posts: 309
From the cisco prep practice exams a question says use the most efficient addressing scheme and vlsm to configure a serial connection between two routers, which address can be configured on one of the serial interfaces?

A) 192.168.16.63/27
B) 192.168.16.158/27
C) 192.168.16.192/27
D) 192.168.16.113/30
E) 192.168.16.145/30
F) 192.168.16.193/30

They say F is correct, why wouldn't D and E be correct as well?

Comments

  • NetstudentNetstudent Member Posts: 1,694
    good question. icon_scratch.gif
    Looks to me like 145 is in the 144 subnet.
    There is no place like 127.0.0.1 BUT 209.62.5.3 is my 127.0.0.1 away from 127.0.0.1!
  • r_durantr_durant Member Posts: 486
    Hmmmmm...good question!! From what I calculated, D, E & F seem to be valid addresses in the scheme of the question asked... icon_scratch.gif
    CCNA (Expired...), MCSE, CWNA, BSc Computer Science
    Working on renewing CCNA!
  • Paul BozPaul Boz Member Posts: 2,621 ■■■■■■■■□□
    Idiologically speaking, you want to reserve /30 masks for point to point links, as they only afford two usable ip addresses, which means absolutely no waste with your IP allocation. That should answer r_durant's question. [edit] r_durant edited his reply. For the sake of anyone that doesn't know this, he asked what the point is in using a /30 over the other choices, since the others would technically work.

    As far as the original poster:

    A) 192.168.16.63/27
    B) 192.168.16.158/27
    C) 192.168.16.192/27
    D) 192.168.16.113/30
    E) 192.168.16.145/30
    F) 192.168.16.193/30

    A, B, and C are instantly out because using wasteful addressing space is against Cisco best practice (and common sense).

    Between D, E, and F,

    D is correct because 192.168.16.113 is a part of the 112 subnet. It breaks down like this:

    192.168.16.112 is the subnet
    192.168.16.113-114 are the valid usable host addresses
    192.168.16.115 is the broadcast address
    192.168.16.116 is the next subnet.

    E is correct because 192.168.16.145 is a part of the 144 subnet. It breaks down like this:

    192.168.16.144 is the subnet
    192.168.16.145-146 are the valid usable host addresses
    192.168.16.147 is the broadcast address
    192.168.16.148 is the next subnet.

    F is correct because 192.168.16.193 is a part of the 192 subnet. It breaks down like this:

    192.168.16.192 is the subnet
    192.168.16.193-194 are the valid usable host addresses
    192.168.16.195 is the broadcast address
    192.168.16.196 is the next subnet
    CCNP | CCIP | CCDP | CCNA, CCDA
    CCNA Security | GSEC |GCFW | GCIH | GCIA
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  • r_durantr_durant Member Posts: 486
    Paul Boz wrote:
    That should answer r_durant's question. [edit] r_durant edited his reply. For the sake of anyone that doesn't know this, he asked what the point is in using a /30 over the other choices, since the others would technically work.

    I did?? icon_scratch.gif

    I agreed with D, E & F... :D
    CCNA (Expired...), MCSE, CWNA, BSc Computer Science
    Working on renewing CCNA!
  • MikdillyMikdilly Member Posts: 309
    It'd be helpful if they could give an explanation for their answers within the practice exam. Was thinking of submitting this question to them, would i get an answer?
  • PashPash Member Posts: 1,601 ■■■■■□□□□□
    I totally agree Mikdilly, questions should always be backed up with explanations. Ever done a google search looking for a topic that is quiet hard to find specific information on...you find a post by some chap somewhere who has EXACTLY the same issue/problem you had....you follow his thread down to the end where he kindly puts.....got it working, thanks everyone for your help......no explanation on how. Yup, i bet most have.

    So please, always always post an explanation to a an issue or problem, just like paul did for us. Back on topic.....yes that question is wrong and yup you probably should let them know.
    DevOps Engineer and Security Champion. https://blog.pash.by - I am trying to find my writing style, so please bear with me.
  • Paul BozPaul Boz Member Posts: 2,621 ■■■■■■■■□□
    r_durant wrote:
    Paul Boz wrote:
    That should answer r_durant's question. [edit] r_durant edited his reply. For the sake of anyone that doesn't know this, he asked what the point is in using a /30 over the other choices, since the others would technically work.

    I did?? icon_scratch.gif

    I agreed with D, E & F... :D

    Hmm, could have sworn you did. Oh well, at any rate it's still good info :)
    CCNP | CCIP | CCDP | CCNA, CCDA
    CCNA Security | GSEC |GCFW | GCIH | GCIA
    [email protected]
    http://twitter.com/paul_bosworth
    Blog: http://www.infosiege.net/
  • MikdillyMikdilly Member Posts: 309
    Sorry, my mistake, there was a graphic on the ques that showed the 2 routers with subnets of 192.168.16.64/27, 192.168.16.96/27, and 192.168.16.128/27. Answers D and E would fall into those ranges. I thought since they used a diff mask they wouldn't apply.
  • NetstudentNetstudent Member Posts: 1,694
    ya be very careful of overlaps when using VLSM and manual route summarization.
    There is no place like 127.0.0.1 BUT 209.62.5.3 is my 127.0.0.1 away from 127.0.0.1!
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