Possible double NAT detected Error message

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  1. #11

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    Address Resolution Protocol does not operate on layer 3 of the OSI, its on the datalink - layer 2.
    thus, operating on LAN. so, when the PC does a ARP request has nothing to do with the router. a PC's dynamic address is MAC.


  2. #12

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    After battling through the really appalling English in that attempt to make a point I'm reasonably sure that you don't understand how ARP caching works. But I'm a bit rusty on this since it's been many years since I studied it, so I'd welcome corrections from people who can explain in coherent English if I have gone wrong.

    As I understand it, ARP caches in a LAN exist for each physical port and contains the MAC address and corresponding IP address of the other end of each link. Each port (WAN and LAN) on the router has a different MAC address (and a separate ARP cache). If you simply unplug a cable from one port on the router and put it into another port whilst all the devices are running then the ARP cache on the PC (or upstream bridge) will contain a MAC address which no longer matches the physical connection, and the ARP cache on the new router port will either have an invalid MAC for the far end of the link (if something else was recently unplugged from it) or an empty cache if nothing has been plugged into that port since the device was started.

    Depending on what type of OS, routers & bridges you have they may or may not deal with this automatically. If they don't then rebooting the router/bridges will clear all the ARP caches and cause them to be repopulated with the correct MAC addresses for whatever is then plugged into each port.


  3. #13

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    Very well said.


  4. #14

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    Quote Originally Posted by Fenix2:
    Very well said.
    do YOU even know how MAC addressing works?

  5. #15

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    Quote Originally Posted by PDLM:
    After battling through the really appalling English in that attempt to make a point I'm reasonably sure that you don't understand how ARP caching works. But I'm a bit rusty on this since it's been many years since I studied it, so I'd welcome corrections from people who can explain in coherent English if I have gone wrong.

    As I understand it, ARP caches in a LAN exist for each physical port and contains the MAC address and corresponding IP address of the other end of each link. Each port (WAN and LAN) on the router has a different MAC address (and a separate ARP cache). If you simply unplug a cable from one port on the router and put it into another port whilst all the devices are running then the ARP cache on the PC (or upstream bridge) will contain a MAC address which no longer matches the physical connection, and the ARP cache on the new router port will either have an invalid MAC for the far end of the link (if something else was recently unplugged from it) or an empty cache if nothing has been plugged into that port since the device was started.

    Depending on what type of OS, routers & bridges you have they may or may not deal with this automatically. If they don't then rebooting the router/bridges will clear all the ARP caches and cause them to be repopulated with the correct MAC addresses for whatever is then plugged into each port.
    what course did you study, then?

  6. #16

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    Quote Originally Posted by protocl:
    what course did you study, then?
    Computer Science (Hons), University of Cambridge. You?

  7. #17

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    Quote Originally Posted by PDLM:
    Computer Science (Hons), University of Cambridge. You?
    Information and Computer Scienece - University Of California, Irvine.
    Cisco Systems - 640/802 CCNA.
    Cisco Systems - CCNP/security, preparing for test.

  8. #18

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    Quote Originally Posted by PDLM:
    After battling through the really appalling English in that attempt to make a point I'm reasonably sure that you don't understand how ARP caching works. But I'm a bit rusty on this since it's been many years since I studied it, so I'd welcome corrections from people who can explain in coherent English if I have gone wrong.

    As I understand it, ARP caches in a LAN exist for each physical port and contains the MAC address and corresponding IP address of the other end of each link. Each port (WAN and LAN) on the router has a different MAC address (and a separate ARP cache). If you simply unplug a cable from one port on the router and put it into another port whilst all the devices are running then the ARP cache on the PC (or upstream bridge) will contain a MAC address which no longer matches the physical connection, and the ARP cache on the new router port will either have an invalid MAC for the far end of the link (if something else was recently unplugged from it) or an empty cache if nothing has been plugged into that port since the device was started.

    Depending on what type of OS, routers & bridges you have they may or may not deal with this automatically. If they don't then rebooting the router/bridges will clear all the ARP caches and cause them to be repopulated with the correct MAC addresses for whatever is then plugged into each port.
    Quote Originally Posted by PDLM:
    Computer Science (Hons), University of Cambridge.
    Quote Originally Posted by protocl:
    Address Resolution Protocol does not operate on layer 3 of the OSI, its on the datalink - layer 2.
    thus, operating on LAN. so, when the PC does a ARP request has nothing to do with the router. a PC's dynamic address is MAC.
    Quote Originally Posted by protocl:
    Information and Computer Scienece - University Of California, Irvine.
    Cisco Systems - 640/802 CCNA.
    Cisco Systems - CCNP/security, preparing for test.
    So there you go folks - pay your money and make your choice...

  9. #19

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    Quote Originally Posted by protocl:
    do YOU even know how MAC addressing works?
    hahaha.... I think stuidied the ARP protocol about 20 years ago...
    I'm a networking old timer I started with IOS 9.x on a Cisco IGS.

    Let me know if you need any used Cisco equipment for your studies.

    and bTW who even bothers talking about the OSI model... :-)
    Last edited by Fenix2; 11-02-2011 at 12:14 PM.

  10. #20

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    Quote Originally Posted by Fenix2:
    hahaha.... I think stuidied the ARP protocol about 20 years ago...
    I'm a networking old timer I started with IOS 9.x on a Cisco IGS.

    Let me know if you need any used Cisco equipment for your studies.

    and bTW who even bothers talking about the OSI model... :-)
    thank you for the offer on the Cisco units, but id rather go the GNS3 route.
    GNS3 utilizes only valid IOS's from cisco systems without the need to buy or use actual units but of course you knew about GNS3, right?

    about the OSI model; just to help illustrate where ARP operates. since ARP, is on the low end of IP to MAC translation.
    Last edited by protocl; 11-02-2011 at 01:20 PM. Reason: add.