Abstract: To resolve the problem that the route list in core switches needs explosive capacity,a CIDR(classless inter domain routing) structure is introduced.The CIDR list efficiently reduces route list capacity,but is not satisfied with the IP address lookup algorithm.With an analysis of binary lookup algorithm based on the prefix length of Hash address,it can be seen that the advantage of this algorithm is not embodied since the limitation of store capacity on CIDR list.With the improvement of this algorithm by the method of prefixes length expansion for a few fixed point addresses,a faster lookup matching algorithm is brought forward which satisfies the requirements of the CIDR list.From the simulated experiment,the improved algorithm meets the demand for fast forwarding IP blocks better.