A relay server, comprising: an address filter information storage unit adapted to store a first routing target address and a second routing target address, the first routing target address being an address of a first routing target device, to which a packet is transferred, located in a first LAN, the second routing target address being an address of a second routing target device to which a second relay server located in a second LAN transfers a packet; a virtual address allocation relationship storage unit adapted to store the first routing target address and a virtual address allocated to the first routing target address while correlated with each other; a virtual address setting information storage unit adapted to store with respect to the second relay server whether communication is conducted using the virtual address or the first routing target address; and a controller adapted to store with respect to the second relay server whether the communication is conducted using the virtual address or the first routing target address in the virtual address setting information storage unit, allocate the virtual address to the first routing target address and store an allocation relationship between the virtual address and the first routing target address in the virtual address allocation relationship storage unit in case the virtual address setting information storage unit stores setting information permitting the allocation of the virtual address, transmit the virtual address allocated to the first routing target address to the second relay server and receive the second routing target address from the second relay server, to establish a routing session with the second relay server, refer to the virtual address allocation relationship storage unit when receiving a packet having a destination of the virtual address from the routing session and convert the destination address of the packet into the first routing target address corresponding to the virtual address, to transfer the packet to the first routing target device of the destination refer to the virtual address allocation relationship storage unit when receiving a packet having a destination of the second routing target device from the first routing target device and convert a source address of the packet into the virtual address allocated to the first routing target address, to transfer the packet to the routing session. an address filter information storage unit adapted to store a first routing target address and a second routing target address, the first routing target address being an address of a first routing target device, to which a packet is transferred, located in a first LAN, the second routing target address being an address of a second routing target device to which a second relay server located in a second LAN transfers a packet; a virtual address allocation relationship storage unit adapted to store the first routing target address and a virtual address allocated to the first routing target address while correlated with each other; a virtual address setting information storage unit adapted to store with respect to the second relay server whether communication is conducted using the virtual address or the first routing target address; and a controller adapted to store with respect to the second relay server whether the communication is conducted using the virtual address or the first routing target address in the virtual address setting information storage unit, allocate the virtual address to the first routing target address and store an allocation relationship between the virtual address and the first routing target address in the virtual address allocation relationship storage unit in case the virtual address setting information storage unit stores setting information permitting the allocation of the virtual address, transmit the virtual address allocated to the first routing target address to the second relay server and receive the second routing target address from the second relay server, to establish a routing session with the second relay server, refer to the virtual address allocation relationship storage unit when receiving a packet having a destination of the virtual address from the routing session and convert the destination address of the packet into the first routing target address corresponding to the virtual address, to transfer the packet to the first routing target device of the destination refer to the virtual address allocation relationship storage unit when receiving a packet having a destination of the second routing target device from the first routing target device and convert a source address of the packet into the virtual address allocated to the first routing target address, to transfer the packet to the routing session.