We present SNAP (Safe and Nimble Active Packets), a new scheme for programmable (or active) packets centered around a new lowlevel packet language. Unlike previous active packet approaches, SNAP is practical: namely, adding significant flexibility over IP without compromising safety and security or efficiency. In this paper we show how to compile from the well-known active packet language PLAN [7] to SNAP, showing that SNAP retains PLAN’s flexibility; give proof sketches of its novel approach to resource control; and present experimental data showing SNAP attains performance very close to that of a software IP router.