How can the number of peers from the same IP address be counted? A Bitcoin client connects to peers and can accept connections from peers.  Is it possible to learn how many nodes exist from a specific IP address?

Specifically, the monitoring service from RowIT ( http://bitcoinstatus.rowit.co.uk ) gives a metric "Total Hosts", but I suspect that if there was more than one host from an IP address, the second and successive hosts would not be counted in the total.
That monitoring page states that it counts hosts from 'addr' messages seen on the bitcoin network. 

An 'addr' message contains the IP number &amp; port number of a bitcoin client. Since all bitcoin clients use the same listening port number the IP number is the only information to count by. 

When a bitcoin client connects to a peer it does send out an extra number to identify it by. However there is no friendly way to force all clients behind an IP number to connect to you so you cannot capture that number for all those clients.