Simulation of the Internet has long been understood to be very challenging mostly because of its scale, diversity and the lack of detailed knowledge of many of its components. However, two recent developments (macroscopic topology discovery and large memory servers) mean that some of these problems are now more tractable. Although problems like the lack of detailed link information remain, models are are useful for some problems that require an understanding of how an application interacts with the Internet as a whole. The paper presents is-0, an Internet Simulator. is-0 derives its model of Internet topology directly from the output of an Internet topology mapping project. Efficient design allows is-0 to simulate packet-by-packet, hop-by-hop behaviour at Internet scale. Validation of is-0, an example application and performance measurements are included.