Chapter 10 IPsec VPN : Hub-and-spoke configurations : Protected networks addressing
  
Protected networks addressing
The addresses of the protected networks are needed to configure destination selectors and sometimes for security policies and static routes. The larger the number of spokes, the more addresses there are to manage. You can
Assign spoke subnets as part of a larger subnet, usually on a new network
or
Create address groups that contain all of the needed addresses