How is this possible (ISP subnet issue)?

--chris----chris-- Member Posts: 1,518 ■■■■■□□□□□
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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?


Comments

  • networker050184networker050184 Mod Posts: 11,962 Mod
    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.
    An expert is a man who has made all the mistakes which can be made.
  • xnxxnx Do they matter? UKMember Posts: 464 ■■■□□□□□□□
    @networker050184 I've been assigned a .255 public ip address, any idea how that's possible??
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  • networker050184networker050184 Mod Posts: 11,962 Mod
    Same way as I said before! It's a part of PPP.
    An expert is a man who has made all the mistakes which can be made.
  • Dieg0MDieg0M Member Posts: 861
    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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  • xnxxnx Do they matter? UKMember Posts: 464 ■■■□□□□□□□
    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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  • theodoxatheodoxa Member Posts: 1,340 ■■■■□□□□□□
    xnx wrote: »
    @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.
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  • --chris----chris-- Member Posts: 1,518 ■■■■■□□□□□
    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


  • networker050184networker050184 Mod Posts: 11,962 Mod
    What are you not understanding? Do you know how PPP works at L2? How the PPP peer route works at L3?
    An expert is a man who has made all the mistakes which can be made.
  • --chris----chris-- Member Posts: 1,518 ■■■■■□□□□□
    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.


  • networker050184networker050184 Mod Posts: 11,962 Mod
    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!
    An expert is a man who has made all the mistakes which can be made.
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