IP caching has been time-proven to be capable of offloading routing-lookup modules. However, in evaluating the performance of IP caching, the cache size is a dominant factor. In this paper, we propose a caching scheme, named aligned-prefix caching (APC). APC takes advantage of prefix caching to reduce the cache size requirement. However, APC requires a routing-lookup module that can indicate whether an aligned prefix is cacheable or not. Thus, we propose a marking scheme, named aligned-ancestor poisoning, for tree-based routing tables. Our trace-driven simulations show that aligned-24 prefix caching can reduce the cache size requirement by about 50%, compared with the conventional destination caching scheme.