Virtual IP Settings for failover

I've just started looking at the above, following the document 'Step-by-Step Guide to
Synchronous Volume Replication...'.

I don't understand the settings for the Virtual IP:

'Error
Virtual IP that was entered cannot be in the same subnetwork as one of the network interfaces or virtual IP that already exist in the system. You have to enter IP that have a unique subnetwork class and is not used in any other place in your configuration.'

What if I don't have another subnet available?. Why does it have to be on a different network to all the other interfaces on the system?

thanks for any help