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: 1,693 ■■■□□□□□□□
    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: 2,620 ■■■■■■■■□□
    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
    pbosworth@gmail.com
    http://twitter.com/paul_bosworth
    Blog: http://www.infosiege.net/
  • 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: 1,600 ■■■■■□□□□□
    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: 2,620 ■■■■■■■■□□
    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
    pbosworth@gmail.com
    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: 1,693 ■■■□□□□□□□
    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!
Sign In or Register to comment.