An IP packet relay apparatus provided between networks to relay an IP packet relating to a host belonging to a specific group, comprising: a list in which an IP address possessed by a host is associated with a group identifier for identifying a group to which the host belongs; a judgment unit judging whether an IP packet that has received from one network and is addressed to a host connected to another network is an IP packet relating to the same group or not with reference to the list; and a forwarding unit relaying an IP packet judged to be the IP packet relating to the same group.