To reduce burden on an operation of delivering a security policy.

In a communication system having a host to be connected to a network and a server having a destination address being unknown by the host, the server is provided with a first storage means for storing data of the security policy required for communication in the network; a request message receiving means for receiving a request message for requesting transmission of the data of security policy, being intended for a first multicast address capable of being received by the server; and a notification message transmitting means for transmitting a notification message including the data of security policy responding to the request message received by the first receiving means to a second multicast address capable of being received by the host. The host is provided with a request message transmitting means for transmitting a request message to a first multicast address; a notification message receiving means for receiving the notification message; and a second storage means for storing the data of security policy included in the received notification message.