To allow the device to retrieve a network in which a destination of a received packet is in existence at a high speed among interconnected networks.

Network addresses are classified into 'large (L), small (S) and inclusive (I)', and an address table of a tree structure where three branches are connected to each node is generated based on the classification. Then a bit length of a mask of a prescribed entry A in the address table is selected to be ma, let the value of high-order ma-bits of a network address of the entry A be a(ma), and the value of the high-order ma-bits of a destination address C be c(ma), then a router retrieves addresses according to the following rules; rule (1) I, S are not retrieved in the case of a(ma)<c(ma), rule (2) L, S are not retrieved in the case of a(ma)=c(ma), and rule (3) L, I are not retrieved in the case of a(ma)>c(ma).