In this paper the authors outline the underlying theory of Internet advertising exposure in today's ad server world. That theory is reduced to a formula called the Probability Model by Exposure Class (PMEC).While helpful in informing thinking about advertising campaigns, the PMEC formula is not practical in day-to-day use. The authors then outline a simulation system based on consumer panel data that was used to build a pragmatic model which produces nearly identical estimates as that produced by the PMEC, but is easier to use in a fully developed Internet reach and frequency planning system. Results from the new model, Coffey-Mazumdar Internet Reach &Frequency model (CMIRF), are compared to those of the PMEC model. Last, a summary of learnings and an outline of potential applications are provided.