A network system is provided. The network system includes a server, a switch connected to a server, and a controller controlling the functions of the server and the switch. The server includes a processor and a network adapter connected to a processor. The network adapter processes packets of each flow in accordance with a filter table. The processor includes a virtual machine and a virtual switch which relays packets transmitted and received by the virtual machine to and from an external entity. The network adapter has an offloading function which exchanges packets with the virtual machine without using the virtual switch. In a tunneling process, the switch exchanges packets with the virtual machine through a tunneling-dedicated port, and the network adapter forwards the packets between the switch and the virtual machine without referring to the filter table. The controller dynamically controls enabling/disenabling of the tunneling process for each flow.