I got following routing setup:

Target        Gateway      Mask

Router connects to internet. Router connects to another LAN. Routing works and I can reach both internet and hosts at 192.168.2.* as expected.


router has changed its IP address to Routing tables has not been changed, they are just like above.

ARP tables has both (old) and (new) if that could have any effect?

That behavior was confusing so I did try same routing configuration with another machine (that's not been part of network before) and as I expected it did not work. Only found from ARP tables as resolved record, which is also expected behavior.

So, my question is: Why does routing to 192.168.2.* hosts still work? There's no valid route to that network anymore.

OS which still connects through "legacy" route is linux 3.18.7 if that matters.

1 Answer 1


It works because the arp entry for is still valid. Your router determines that the next hop is and looks in the arp cache. Since the entry is still there, if sends the frame to that MAC address, which is still your router.

If you wait a while, the entry will age out and the network will be unreachable. The new computer you connected didn't have the old entry, so that's why it didn't work.

This article explains how the arp cache ages out on Linux machines.

  • 1
    In short, while there's active connections to foreign network (and there is) arp entry stays alive until ethernet gets disconnected/reconnected. I don't know how long it's been since router ip has changed but routing still holds atow. Dec 19, 2015 at 12:29

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