A cuckoo filter is a highly efficient data structure that provides approximate set-membership testing and addresses some drawbacks of a Bloom filter such as deletion and space overhead. Motivated by the application of Bloom filters to IP lookup, and in general, fast packet processing at routers, we propose a new filter called a length-aware cuckoo filter (LACF), for faster IP lookup with limited extra storage requirement. LACF uses different numbers of hash functions to store and search for entries based on the prefix length popularity of routing entries. Our preliminary results show that LACF reduces the false positive rates for IP lookup by a factor of up to two, with less than 1% storage increase. As future work, we will implement LACF in real network routers, and validate its effectiveness as an IP packet forwarding mechanism.