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Forum Replies

  1. Hi Mario,

    The first two bits have to be 10 so that means the following range is class B:

    10000000 - 10111111

    Or in decimal:

    128 - 191

    Rene

  2. Hi Rene,

    First of all thanks for making all topics easy with nice explanation.

    I have some doubt on this prefix-list even after going through all Topic and QnA.

    First,
    How Prefix-list differentiate between Network and Host. Does it differentiate like ACL or deal these two in different more suitable manner.
    Like in ACL if I am writing permit 10.0.0.0/8 it includes 10.0.0.0 to 10.255.255.255. Is it same in Prefix-list ?

    Second,
    As prefix-list says it is exact match.
    ip prefix-list test1 permit 10.0.0.0/8
    will it allow only a single IP 10.0.0.0 like in ACL HOST 10.0.0.0 command does or it will allow 10.0.0.0 to 10.255.255.255 like ACL 10.0.0.0/8 does.

    Third,
    ip prefix-list test2 permit 10.0.0.0/16 ge 24
    In first condition it matches 1st and 2nd actate, second condition it matches /24, /25 ,/26 , /27 , /28 , /29 , /30 , /31 , /32 ignoring any value (0-255 ) in fourth octate. How it treats with the value of 3rd octate , does it deny any value between 1-255.

    Fourth,
    ip prefix-list test3 permit 0.0.0.0/0 le 32 - It says permit any route
    ip prefix-list test4 permit 0.0.0.0/0 - It says permit default route
    Default route it self says permit any route so why it is different here in command.

    Fifth,
    If I want to permit 192.0.0.0 IP only then , will I write 192.0.0.0/2 or 192.0.0.0/8, what is difference between two.
    ip prefix-list test5 permit 192.0.0.0/2 ge 23 le 24
    ip prefix-list test5 permit 192.0.0.0/8 ge 23 le 24
    will these two Prefix-list serves the same output, if not then what are the IPs covered separately in these two.

    Thanks in advance...

  3. Ok, Hi everyone just joined the site.

    I want to make sure that my brain understands this. It's like you slice and dice and hopefully everything comes out right. So say I have this already as my prefix list:

    10.0.12.0/24 le 32

    Now say I want to include another network. So I want the following two networks in one prefix-list.

    10.0.12.0/24 and 10.0.13.0/24, I want these covered by one prefix list.

    So I figure that this will fit within the scope of these two networks.

    2 will be size of subnets:

    0, 2, 4, 6, 8, 10
    12 and 13 --- this fits just right.
    14, 16, 18 etc.....

    So I delete the old prefix and add this:
    10.0.12.0/23 le 32

    I'm not worried about the "le 32" as that basically means I'm accepting all addresses in the 10.0.12.x and 10.0.13.0 scope.
    So do I understand this correctly?

    Thank you!

  4. Hi Michael,

    Seems you got it right yes:

    10.0.12.0/24 le 32

    This will match all 1.0.12.X networks that have a subnet mask of /32 or larger (like /31, /30, /29, etc.).

    With this one:

    10.0.12.0/23 le 32

    You have everything that falls within 10.0.12.0/23 range and with a subnet mask larger than /32 (/31, /30, /29, etc.).

    Rene

  5. I was so confused by this at first. I know its simple but its also a brain teaser for some reason.
    first I pulled up my boson subnet tool its free tool by the way on the Boson.com (just need to create an account) website. At first I was thinking that first two bits mean the first two spots _ _ thinking it could be anything from 0 to 192 that it did not matter if it was a zero or a one. However after putting into the subnet calculator it helped me to see.

    The next really helpful thing for me was when I went ahead and enabled the command with distribute-list prefix CLASSB in all of a sudden all my 10.x.x.x networks disappeared which shot my first theory to crap along with supporting the boson subnet calculator.

    So seeing it in play in a lab really made sense. I think the confusion goes back to the rule on how the classes are setup. When we was learning sub-netting classes that you read and say oh ok but as time goes on you just get use to seeing the numbers themselves and that they are a certain class.

    I am betting everyone that had a problem with this does not use the Class A, B, C rule anymore but instead over time have subconsciously just memorized the 1-127 is A, 128-191 is B, 192-223 is C and does not really think of the rule about class A the first bit always being 0, and class b the first two bits being set to 10, and class C having its first three bits set to 110… (hoping me explaining this in writing will actually help me remember it! lol)

    So its like trying to do a math problem and finally that silly rule in math never used much is key to how the expression functions.

    anyway below is some more information reinforcing Rene info.

    posted the rule below in greater detail that Rene implicitly mentioned briefly in his post I am one of those type that can sometimes be slow seeing something the way it should be seen until I experience it for myself.
    https://www.tutorialspoint.com/ipv4/ipv4_address_classes.htm

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