Server taken offline by other using same IP

Should this be possible?

[inbound]

It looks as though a development server of ours has been unavailable due to another server using the same IP address (meaning we could not access our server and saw the default page for the offending server when we entered the IP of it).

The question I have is simple:

Should this be possible if a hosting company has taken reasonable care?

The company in question has taken the offending server offline and says there is nothing to stop another server on the network from causing the same issue.

I don't know that much about networks but I would guess that the a properly configured router in the rack should only allow traffic for a specific IP address to be sent to the machine that it has been told is allocated that IP. Surely not having this functionality is leaving their system wide open to abuse?

Any thoughts on this would be greatly appreciated.

[phranque]

it is quite normal to have multiple servers at a single ip address, so that in and of itself is not a problem.

[mcavic]

That is true, if you bring up a new machine with the same IP as an existing machine on the same LAN, they will conflict.

There isn't any real defense that I know of. It's the Ethernet switch, rather than the router, that should care, and theoretically it could only grant specific IPs to specific MAC addresses, but I don't think I've heard of any that do that. I have a server colocated in a very high-class data center, and in my contract, it says that I can be terminated for causing IP address conflicts.