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The paper presents an IP address lookup algorithm with low memory requirement and fast updates. The scheme, which is denoted prefix-tree, uses a combination of a trie and a tree search, which is efficient in memory usage because the tree contains exactly one node for each prefix in the routing table. The time complexity for update operations is low for the prefix-tree. The lookup operation for the basic binary prefix-tree may require that a high number of nodes be traversed. The paper presents improvements to decrease lookup time, including shortcut tables and multi-bit trees. The prefix-tree is compared to a trie and a path compressed trie using prefixes from a real routing table.