Dynamic Custom Properties by subnet

I have created a custom property called Site and want to add my nodes to a list of 7 sites.   Is there a way to add a subnet as a whole so if a new node is added to a subnet its added to the site?   I've only been able to add the nodes individually.  I can query for 10.1.3. and get a list of 6 nodes .1 - .6 and select all six to add at once but I want .7 and so on to be added automatically when they are created.   Or at least just add the whole subnet now so I don't have to select each node.   Is there a way to do either?