Processing packet headers

A network interface device for providing an interface between a host device and a network by receiving packets over the network and passing at least some of those packets to ports of the host device, each packet comprising a control section having one or more fields indicative of the type and data protocol of the packet, a source address field indicative of the source address of the packet, a destination address field indicative of the destination address of the packet, a source port field indicative of the source address of the packet and a destination port field indicative of the destination address of the packet; the network device comprising: a data store for storing specifications for packets that are to be passed to the host device, each specification comprising first, second and third check fields; and a packet selection unit for selecting in accordance with the content of the data store which packets received over the network are to be passed to the host device; the packet selection unit being capable of identifying the protocol of a received packet and operable in at least: a first mode in which for packets of a first protocol and of a type indicative of a request to establish a new connection it passes such packets to the host device only if the data store stores a specification whose first check field matches the destination address of the packet, whose second check field matches a reserved datagram and whose third check field matches the destination port of the packet; and a second mode in which for packets of a second protocol it passes such packets to the host device only if the data store stores a specification whose first check field matches the destination address of the packet, whose second check field matches the destination port of the packet and whose third check field matches the reserved datagram.