This paper describes the basic principles of designing and administering a relatively secure network. The principles are illustrated by describing the security issues a hypothetical company faces as the networks that support its operations evolve from strictly private, through a mix of Internet and private nets, to a final state in which the Internet is finally integrated into its operations and the company participates in international electronic commerce. At each stage, the vulnerabilities and threats that the company faces, the countermeasures that it considers, and the residual risk the company accepts are noted. Network security policy and services are discussed, and a description of Internet architecture and vulnerabilities provides additional technical detail underlying the scenario. Lastly, a number of building blocks for secure networks are presented that can mitigate some of the vulnerabilities.