For efficiency and portability, network packet processing code is typically written in low-level languages and makes use of bit-level operations to compactly represent data. Although packet data is highly structured, low-level implementation details make it difficult to verify that the behavior of the code is consistent with high-level data invariants. We introduce a new approach to the verification problem, using a high-level definition of packet types as part of a specification rather than an implementation. The types are not used to check the code directly; rather, the types introduce functions and predicates that can be used to assert the consistency of code with programmer-defined data assertions. We describe an encoding of these types and functions using the theories of inductive datatypes, bit vectors, and arrays in the CVC3 SMT solver. We present a case study in which the method is applied to open-source networking code and verified within the Cascade verification platform.