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This paper deals with the traditional IP address lookup problem with fast updates. We propose a B-tree data structure, called MMSPT (multiway most specific prefix tree), which is constructed by using the most specific prefixes in routing tables. MMSPT arranges the most specific prefixes as the keys in B-tree. Unlike the previous schemes, every prefixes in routing tables is stored exactly once in our MMSPT. For a routing table of n prefixes, MMSPT requires O(n) memory, and the time for search, insertion and deletion operations are O(logmn), O(mlogmn), and O(mlogmn), respectively (m is the order of the B-tree). Our experimental results conducted by using five real IPv4 routing tables show that MMSPT outperforms two existing B-tree data structures, PIBT (prefix in B-tree) and MRT (multiway range tree), in all aspects. Moreover, since the complexities of MMSPT is not subject to the length of IP addresses, the proposed MMSPT can be easily extended to fit the IPv6.