The first age of Internet architectural thinking concentrated on defining the correct principles for designing a packet-switched network and its application protocol suites. Although these same principles remain valid today, they do not address the question of how to reason about the evolution of the Internet or its interworking with other networks of very different heritages. This paper proposes a complementary methodology, motivated by the view that evolution and interworking flexibility are determined not so much by the principles applied during initial design, but by the choice of fundamental components or "design invariants" in terms of which the design is expressed. The paper discusses the characteristics of such invariants, including examples from the Internet and other networks, and considers what attributes of invariants best support architectural flexibility.