A network system includes: a core switch; and an edge switch. The edge switch includes: a join message identification unit; and a marking unit. The join message identification unit identifies a join message from among MAC frames from the user network. The marking unit marks mark information to a header of a MAC-in-MAC frame in which the identified join message is encapsulated. The core switch includes: a plurality of input/output ports; a mark identification unit; and a port setup unit. The mark identification unit identifies a MAC-in-MAC frame in which a header is marked with the mark information. The port setup unit associates a multicast group of a join message which is encapsulated in the identified MAC-in-MAC frame, with an input/output port to which the identified MAC-in-MAC frame is input.