A Abstract: When packet filtering is used as a security mechanism, different routers may need to cooperate to enforce the desired security policy. It is difficult to ensure that they will do so correctly. We introduce a simple language for expressing global network access control policies of a kind that filtering routers are capable of enforcing. We then introduce an algorithm that, given the network topology, will compute a set of filters for the individual routers; these filters are guaranteed to enforce the policy correctly. Since these filters may not provide optimal service, a human must sometimes alter them. A second algorithm compares a resulting set of filters to the global network access control policy to determine all policy violations, or to report that none exist. A prototype implementation demonstrates that the algorithms are efficient enough to give quick answers to questions of realistic scale.