This is one of the small things that sometimes drag my attention.
A customer once asked me if it was possible to create a subnet in Active Directory that would catch all clients, and then still have some individual subnets defined for exceptions.
I must confess that I didn’t know so I decided to try, but have never had a chance to report the result on the blog.
I created to sites SiteA and SiteB and created a DC in boot sites, then I created on subnet to catch all clients using ip addresses in 10.*.*.* pointing to SiteA:
Then I created a new subnet for a smaller subnet which was a subset of the catch-all subnet and pointed this to SiteB.
All clients clients on 10.*.*.* used SiteA except clients on 10.5.5.* which used SiteB as I had hoped for.
So it looks like it is possible to have a kind of wildcard subnet to catch all subnet that you might get in the future and decide the default site for those new subnets.
The use of the catch-all subnet will off course depend on your IP-addresses in use, and still require subnets to be created if the default Site is not the right one.