Srinivasan and Varghese [16] have proposed the use of multibit tries to represent routing tables used for Internet (IP) address lookups. They propose an O(k*W²) time dynamic programming algorithm to determine the strides of an optimal k-level multibit fixed-stride trie when the longes prefix in the routing table has length W. We improve on this algorithm by providing an alternative dynamic programming formulation. While the asymptotic complexity of the resulting algorithm for fixed-stride tries is the same as that of the algorithm of [16], experiments using real IPv4 routing table data indicate that our algorithm runs 2 to 4 times as fast.