The Internet's rapid growth has spurred development of new protocols and algorithms to meet changing operational requirements– such as security, multicast transport, mobile networking, policy management, and quality-of-service support. Development and evaluation of these operational tools requires answering many design questions. % Despite their value, custom simulators, wide-area test-beds, and small-scale lab evaluations all have drawbacks. Because they use real code, experiments run in testbeds or labs automatically capture important details that might be missed in a simulation. However, building testbeds and labs is expensive, reconfiguring and sharing them is difficult, and they are relatively inflexible. % In the current paradigm, directly comparable data would be available only if designers implemented all competing mechanisms within every simulator. % The Virtual InterNetwork Testbed (VINT) project, detailed and explained in this article, provides improved simulation tools for network researchers to use in the design and deployment of new wide-area Internet protocols.