on ipsec (openswan/strongswan) you can set the virtual_private value (aka %v4:10.0.0.0/8,%v4:172.16.0.0/12,%v4:192.168.0.0/16,%v4:!192.168.2.0/24,%v4:!192.168.15.128/25) in network - virtual ip settings - virtual subnets. You have to put the string here, it will be directly passed into ipsec.conf for the connection block.
