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  1. Senior Member --chris--'s Avatar
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    #1

    Default How is this possible (ISP subnet issue)?



    The ISP has documented that the mask for this address is /24...?

    How is the GW in a different sub than the public IP? Is there some magical ISP voodoo that I am not aware of that makes me?
    Last edited by --chris--; 10-21-2014 at 07:55 PM.


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  3. Went to the dark side.... Moderator networker050184's Avatar
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    #2
    Yes it is voodoo! By nature of a point to point link like that it really doesn't matter what IP is used. Look at PPP peer routes for an example.
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  4. xnx
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    #3
    @networker050184 I've been assigned a .255 public ip address, any idea how that's possible??
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  5. Went to the dark side.... Moderator networker050184's Avatar
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    #4
    Same way as I said before! It's a part of PPP.
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  6. Network Engineer Dieg0M's Avatar
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    #5
    When PPPoE negotiates using IPCP for dynamic allocations, it installs a /32 host route towards the other end of the PPP link to allow prefix reachability.
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  7. xnx
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    #6
    Thanks guys, it clicked in my head (PPP) after posting but I thought I'd leave the post up. I am so used to /30's being used that I didn't think before asking
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    #7
    Quote Originally Posted by xnx View Post
    @networker050184 I've been assigned a .255 public ip address, any idea how that's possible??
    Depending on the subnet mask, both .255 and .0 could be usable IPs. I had an instructor in College who loved to throw those in on subnetting questions to screw people up. I personally, use a /23 at home.
    Last edited by theodoxa; 10-21-2014 at 10:25 PM.
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  9. Senior Member --chris--'s Avatar
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    #8
    Quote Originally Posted by networker050184 View Post
    Yes it is voodoo! By nature of a point to point link like that it really doesn't matter what IP is used. Look at PPP peer routes for an example.
    This links explains some of it but it still leaves me wondering HOW is this possible?!?


    https://learningnetwork.cisco.com/docs/DOC-10727


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  10. Went to the dark side.... Moderator networker050184's Avatar
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    #9
    What are you not understanding? Do you know how PPP works at L2? How the PPP peer route works at L3?
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  11. Senior Member --chris--'s Avatar
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    #10
    Quote Originally Posted by networker050184 View Post
    What are you not understanding? Do you know how PPP works at L2? How the PPP peer route works at L3?
    I am guessing both since neither of those made me realize how this works, I don't know if the finer points of PPP are covered in the ICND2 topics to be honest. They may be and they just didn't stick.


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  12. Went to the dark side.... Moderator networker050184's Avatar
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    #11
    A PPP frame does not set an address at L2 (well there is a field but it's always set to all 1's) since it is a P2P connection and only one device on the other end. So no worry about addressing there. The PPP peer routes places a /32 route of the other end of the P2P connection as directly connected. So now you have L2 and L3 resolved for forwarding packets regardless of them being on the same subnet or not.

    The answer in networking is always "it depends" as there is a way to work around pretty much everything you learn as a hard rule in the CCNA!
    Last edited by networker050184; 10-23-2014 at 02:23 PM.
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