2 sample summarization subent questions?

pinkiaiiipinkiaiii Posts: 216Member
got one question on practice test basically it was asking for 113 p2p links for wan,and to use address scheme that would preserve as many ips possible,the ip in use was class a 10.0.0.0

thus possible answers were 10.0.0.0/24 /23 /25 /26 -now simple calculations would point to use the /25 thus getting 126 hosts with plenty subnets.as where when checking answer it was /23 which would total to 510 hosts so its way out of preserving ips ?


Second question on summarization asked to summarize networks from 172.16.1.0 to 172.16.13.0

so doing binary

1 = 0000.0001
13 = 0000.0111

thus summary address would be 172.16.1.0 /21 yet answer is /20 reasoning that because since addresses are most likely from different networks of /24 thus need to remove 4 bits ?

thus not sure are sample questions given here are just mistakes,or am i missing something.thx

Comments

  • OctalDumpOctalDump Posts: 1,722Member
    pinkiaiii wrote: »
    got one question on practice test basically it was asking for 113 p2p links for wan,and to use address scheme that would preserve as many ips possible,the ip in use was class a 10.0.0.0

    thus possible answers were 10.0.0.0/24 /23 /25 /26 -now simple calculations would point to use the /25 thus getting 126 hosts with plenty subnets.as where when checking answer it was /23 which would total to 510 hosts so its way out of preserving ips ?


    Second question on summarization asked to summarize networks from 172.16.1.0 to 172.16.13.0

    so doing binary

    1 = 0000.0001
    13 = 0000.0111

    thus summary address would be 172.16.1.0 /21 yet answer is /20 reasoning that because since addresses are most likely from different networks of /24 thus need to remove 4 bits ?

    thus not sure are sample questions given here are just mistakes,or am i missing something.thx

    The answers are correct. For your point to point links, you need two addresses for each end point plus the broadcast address plus the network address. This makes 4 addresses for each link. So each subnet is a /30. But you need 113 of them, which means 113 x 4 = 452 addresses, which means all the addresses from 10.0.0.0 to 10.0.1.195. Which is all of 10.0.0.0/24 plus most of 10.0.1.0/24. You need to round up to the nearest subnet so that becomes 10.0.0.0/23.

    The second one is similar, but you round up to 172.16.16.0 and that's where your 4 bits are 16 = 2^4. Which means the network address uses 4 bits less than a /24 would. So 24-4=20 172.16.0.0/20. Of course that includes other networks (172.16.0.0/24 and 172.16.14.0/23), so might cause problems, but these questions usually don't care about these things.
    2017 Goals - Something Cisco, Something Linux, Agile PM
  • pinkiaiiipinkiaiii Posts: 216Member
    Thank you this sheds some light on why the answers are correct-thus get the point of wan links needing 2 addresses and since block is 4 that 113 needs to be multiplied to meet the criteria even thou only 2 are usable addresses per subnet.

    As per your explanation if such addressing scheme is given where say networks start at say 172.16.1.0-172.16.7.0 = i would need to round it up to 8.0 before doing summary to get correct summarized route prefix ,thus treating address range if it was 16.0.0 to 16.8.0

    have to edit this even per my example if say i use binary to write out network 8 or 16 i still have to draw the line before 16 since difference in bits start at that number and as i understand you always draw a line before difference occurs
    like say 000|0.0001 -1
    000|1.0000 -16 thus i still end up in the wrong since mismatch occurs and theres no 2x0 or 2x1s ?

    anyway sorry for keep editing but think im seeing the picture now you mentioned networks would be included so means since im taking first network and last thus im setting myself up for mistake if i took network 15 or say as per my second example 7 thus i would get correct indication where to draw the line in difference occurring,rather then just relying on firt and last networks.
  • volfkhatvolfkhat Posts: 944Member ■■■■■■■□□□
    pinkiaiii wrote: »
    Second question on summarization asked to summarize networks from 172.16.1.0 to 172.16.13.0

    Here's an alternative approach (forget about the Binary way):
    Look at the two addresses:
    172.16.1.0
    172.16.13.0

    Pretend that these 2 addresses belong to two computers:
    Computer-A = 172.16.1.0
    Computer-B = 172.16.13.0

    Now, pretend that the Question asked:
    "Computer-A and Computer-B are both on the SAME subnet."

    Figure out the Subnet/Mask.
  • volfkhatvolfkhat Posts: 944Member ■■■■■■■□□□
    The easiest answer is:
    Subnet: 172.16.[B]0.0[/B]
    Mask:   255.255.[B]0.0[/B]
    
    And this would be correct.

    However, try and figure out a smaller range :]
  • pinkiaiiipinkiaiii Posts: 216Member
    volfkhat wrote: »
    The easiest answer is:
    Subnet: 172.16.[B]0.0[/B]
    Mask:   255.255.[B]0.0[/B]
    
    And this would be correct.

    However, try and figure out a smaller range :]

    Once you put that way it seems actually easy since class b and when you mention addresses are on same subnet then its sort of obvious,dunno if you can give me some random ips to solve ,when you mention smaller range ?
  • james43026james43026 Posts: 303Member
    Technically the practice test is wrong on both questions, as you wouldn't want to create a summarization / supernet that has addresses that don't exist, or even worse, they exist elsewhere in your topology. That just lead to a potential blackhole in your network.
  • OctalDumpOctalDump Posts: 1,722Member
    james43026 wrote: »
    Technically the practice test is wrong on both questions, as you wouldn't want to create a summarization / supernet that has addresses that don't exist, or even worse, they exist elsewhere in your topology. That just lead to a potential blackhole in your network.

    Yeah, that first question doesn't sound like a route summarisation question at all per se. I can't see an instance where you would summarise a whole group of WAN links. It seems more about reserving a range to accommodate a number of point to point links.

    Possibly if you wanted to remotely manage a large section of the network through a particular gateway, you might summarise like this, and that could lead to problems if you've used other IPs in that range for other purposes or other locations.
    2017 Goals - Something Cisco, Something Linux, Agile PM
  • pinkiaiiipinkiaiii Posts: 216Member
    well many questions that i come across dont make sense to me,but well its cisco so you never know what they might throw and consider right answer.

    Anyway here's a similar one i encountered today while practicing.

    192.168.25.0/30 is subnneted 4 subnets.

    192.168.25.20 via 90/34353453
    192.168.25.16 via 90/21344544
    192.168.25.24 via 90/23454565
    192.168.25.28 via 90/34354355

    Q.Which address and mask combination a summary of the routes learned by EIGRP ?

    A.192.168.25.16 255.255.255.252
    b.192.168.25.28 255.255.255.240
    c.192.168.25.0 255.255.255.252
    d.192.168.25.16 255.255.255.240
    e.192.168.25.0 255.255.255.240
    f.192.168.25.28 255.255.255.240

    the metrics used is just random numbers didn't want to type out full length lines,but rest is correct question,how one would approach such question since say you have minutes time to give correct answer and best way to break it down to get understanding ?

    Since when typing out question and knowing answer its sort of easy to see how i would do it now,but once you set yourself for time limit and get trickly worded question with a chart it just throws me off the logical thinking.
  • volfkhatvolfkhat Posts: 944Member ■■■■■■■□□□
    A more specific range would be

    Subnet: 172.16.0.0
    Mask: 255.255.240.0

    ~this would cover the addresses from 172.16.0.0 - 172.16.15.254.
    (which includes the two address you needed to cover)
  • volfkhatvolfkhat Posts: 944Member ■■■■■■■□□□
    pinkiaiii wrote: »
    Once you put that way it seems actually easy since class b and when you mention addresses are on same subnet then its sort of obvious,dunno if you can give me some random ips to solve ,when you mention smaller range ?

    A more specific range would be

    Subnet: 172.16.0.0
    Mask: 255.255.240.0

    ~this would cover the addresses from 172.16.0.0 - 172.16.15.254.
    (which includes the two address you needed to cover)
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