This document discusses flexible routing tables (FRT), a new approach for designing routing algorithms in overlay networks. FRT defines a total order on routing tables based on an identifier assigned to each entry. This allows routing tables to dynamically adjust while maintaining the invariant of the total order. The document outlines the key procedures of FRT, including entry learning and filtering, and evaluates its scientific and engineering impacts. Examples are given of how FRT has been applied to develop new routing algorithms for Chord and XOR-based overlays. In conclusion, FRT provides a unified framework for routing in both full-mesh and multi-hop overlay networks.