A relay server stores a first routing target address, and stores a second routing target address in association with a virtual address assigned to the second routing target address. The relay server exchanges address filter information with a relay server belonging to a selected VPN group, and establishes a routing session with the relay server. The relay server determines whether or not there is an overlap between the routing target addresses. Upon determination that there is an overlap, the relay server assigns a virtual address to the second routing target address. The relay server uses the virtual address to perform communication between the relay server itself and a first routing target device.