The present invention relates to a communications control technology and an information relaying technology intended for a communications network system such as a LAN (Local Area Network). More particularly, the present invention relates to the technology which may be effectively applied to a communication control method intended for an inter-network device such as a LAN switch (Layer 2 switch, Layer 3 switch, or the like) and a communications network system composed of the inter-network devices.
One of the technologies characterized by the LAN switch is a VLAN (Virtual LAN). The VLAN is a technology which makes it possible to build the LAN without depending upon a physical port of the inter-network device. The VLAN may be divided into the following types; a port-based VLAN, a MAC (Media Access Control) address-based VLAN, a Layer 3 protocol-based VLAN, an IP (Internet Protocol) subnet-based VLAN, and the like.
For example, in the communications network system of the IP subnet based VLAN, a LAN switch 210 provided with plural ports 221 to 225 receives packets destined for a PC (Personal Computer) 233 from a PC 231. In response, the LAN switch 210 operates to learn a source MAC address and a source IP address and then create a host table 220. Then, the LAN switch 210 operates to refer to the host table with the destination IP address of the received packet as a key and then, if the host table 220 includes an entry having the corresponding destination IP address to the source IP address, output the packet to the port specified by a port number field of the entry. If such an entry is not found in the host table 220, the LAN switch 210 operates to refer to a routing table (not shown) and an ARP (Address Resolution Protocol) table (not shown) for determining a next hop. Then, the LAN switch 210 operates to create a new entry of the host table 220 and then output the packet to the corresponding port to the next hop. The foregoing procedure is the relaying operation of the LAN switch 210 from the PC 231 to the PC 233.
Further, the LAN switch 210 operates to periodically delete an entry of the host table 220 and newly learn the addresses based on the source and the destination information of the received packet for keeping the entry of the host table 220 being updated. Hence, if the PC is moving to another place, the LAN switch 210 may correctly relay the packets to the moved port. That is, if the PC is moved, the PC may automatically restart the communication in a similar manner to that before movement.