Machine: Xen-3.0 image running stable Debian Linux 2.6.18, pretty vanilla.

My VPS provider asks me to deal with some trouble my image is causing, namely handling IP addresses it is not supposed to:

The problem is that your server seems to be configured to use IPs that have not been appointed to you. Your server responds to ARP requests for the IPs 81.171.111.219 and 81.171.111.218. But you are not allowed to use those.

Not explicitly, as far as I can tell! At least, nothing under /etc or /var/tmp mentions these IP addresses. But arp -v says something I can't make sense of:

Address                  HWtype  HWaddress           Flags Mask            Iface
81.171.111.1             ether   00:0C:DB:E3:80:00   C                     eth0
Entries: 1      Skipped: 0      Found: 1

What is it listening to? The possibilities seem to be:

1. It's not my fault: my VPS providers have overlooked something. What might that be?
2. 81.171.111.1 means I'm happy listening in on ARP requests that I shouldn't be: how do I change this? In any case, what does this mean?
3. I'm looking in completely the wrong place for information on what my image is doing. Where should I be looking?