Problem when swapping two IP addresses machines

[Frank Bonnet]

Hello

Yesterday I swapped the IP addresses of two machines
( new and old mail server ).

After I rebooted the two machines I was unable to join
our DNS server ( even to ping it )

I suspect the DNS was keeping the IP / MAC address mapping
of the two swapped machines , so I flushed the ARP cache but
it was still impossible to join the DNS from the concerned
machines. I have to reboot the DNS Server to be able to
ping it and use it again.

The DNS server runs FreeBSD 8.0-RELEASE-p4.

Thanks for any infos.