A network system includes a server, a first node, a second node, and a plurality of relay nodes. The first node is linked to the server for transmitting data. The second node is linked to the server for receiving data. Each relay node of the plurality of relay nodes is linked to the server for relaying data. When the first node and the second node belong to a configuration of network address translation type 3, a relay node is selected from the plurality of relay nodes. A first data link is established between the first node and the relay node. A second data link is established between the second node and the relay node.