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ACL question

Discussion in 'General Cisco Certifications' started by Stoney, Mar 11, 2008.

  1. Stoney

    Stoney Megabyte Poster

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    Morning,

    I'm just trying to get my head around Access Control lists and the inverse mask using this here page.

    Now it says that these network addresses;

    192.168.32.0/24
    192.168.33.0/24
    192.168.34.0/24
    192.168.35.0/24
    192.168.36.0/24
    192.168.37.0/24
    192.168.38.0/24
    192.168.39.0/24

    Can be summarised into one address: 192.168.32.0/21 or 192.168.32.0 255.255.248.0 to give you the inverse mask of 0.0.7.255.

    This makes sense so far and I think I understand it.

    However, in the next example, these addresses;

    192.168.146.0/24
    192.168.147.0/24
    192.168.148.0/24
    192.168.149.0/24

    Can not be summarised into one network address, you need a minimum of 2 networks. Yet when you write them out in binary the first 5 bits of the third octet are all the same and is the same as the first example.

    I do not understand why you can not put these all in one network! :eek:

    Could somebody explain to me why this is the case?

    Thanks
     
    Certifications: 25 + 50 metre front crawl
    WIP: MCSA - Exam 70-270
  2. sunn

    sunn Gigabyte Poster

    1,562
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    First off, just to make sure we're clear, you've stated ACL (Access Control Lists) in the header but the real question is about summarization.
    OK so let's try this.
    The thing to remember is that route summarization is used to combine a number of routes into fewer statements, thus it takes less memory and processor resources.

    From your example
    192.168.146.0/24
    192.168.147.0/24
    192.168.148.0/24
    192.168.149.0/24

    You're right, the following address does work (it can be used to summarize the addresses):
    Decimal: 192.168.144.0
    Binary: 11000000.10101000.10010000.00000000

    However, it does too much. Let's work out the example:
    11000000.10101000.1001xxxx.0
    'xxxx' - inclussions
    0001 - 145
    --0010 - 146
    --0011 - 147
    --0100 - 148
    --0101 - 149
    0110 - 150
    0111 - 151
    1000 - 152
    1001 - 153
    1010 - 154
    1011 - 155
    1100 - 156
    1101 - 157
    1110 - 158
    1111 - 159

    You'll see that the problem is not only does the summarization cover the routes in the question, but also everything else between:
    192.168.144.0 - 192.168.159.0 That's way beyond the scope of the question.
    What does it mean? Any route between the summarized range will be advertised out this router.

    By using 2 more bits you can summarize without capturing the extra routes:
    11000000.10101000.100100xx. - includes: 192.168.146-147
    11000000.10101000.100101xx. - includes: 192.168.148-149

    Hope that helps...
    It's pretty early for me, so if it doesn't make sense let me know :blink
     
  3. Stoney

    Stoney Megabyte Poster

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    Hey Sunn,

    That's great and makes perfect sense :thumbleft

    The reason I stated ACL's was because that was what I was reading up on when summarisation popped on to the scene. Didn't realise it was a different subject, but now I do it's something else to go and learn!

    Thanks again :biggrin
     
    Certifications: 25 + 50 metre front crawl
    WIP: MCSA - Exam 70-270
  4. sunn

    sunn Gigabyte Poster

    1,562
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    Glad to be of service :biggrin
     

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