IP address lookup engine is the beating heart of a router. For meeting the requirements of a desirable high-speed router, speed, memory consumption, scalability, and reconfigurability of its IP lookup engine are critical. This paper uses a genetic-algorithm approach to optimise the structure of fixed-stride multibit-trie IP lookup methods. In this work, the genetic algorithm is used first in the design phase as an offline optimisation mechanism. This nature-inspired simulation finds the most memory-efficient configuration of IP address segmentation for a fixed number of address segments. Then, for adapting to network variations, the proposed method dynamically changes the number of address segments to compromise between speed and memory consumption. Each time the number of segments changes, an online genetic program is run to optimise the segmentation. Therefore, the lookup engine reconfigures itself to cover more prefixes during the time. The reconfigurability in response to the network variations, and scalability to the number of prefixes improves the life time of a router that uses this method.