By default xen, and all those tools, like Cpanel / WHM and Webmin, don’t really have a way of sorting out ip conflicts. So basically you have some scripts that will setup clients ip address during his machine startup, you have vif ip statment in the config file. But what’s really holds clients from entering:

# ifconfig eth0 x.x.x.x

Where x.x.x.x is the IP of some super important server in same netmask. Luckily for me I came across this problem while still in testing.
Read more .. Avoid IP conflicts in Xen DomU | Planet Admon