IPv6 is often deployed in wireless sensor networks, and the advantages of such a deployment are well reported. For researchers which wish to experiment with new protocols or cross-layer design, there is often a choice of which IPv6 stack to base their work on. The FlexibleIP (FIP) stack presented here is an open-source design which makes documentation, code readability, and ease of modification a priority. In addition FIP provides compile-time options to support a wide variety of devices; it can fit into an 8-bit microcontroller, but can still use features typical when targeting larger microcontroller such as dynamic memory allocation for buffering many packets. The stack also offers excellent support for cross-layer design and hardware acceleration, two areas of special interest for lowpower design. Conformance testing of the stack assures it adheres to relevant standards, and static analysis of the code is also performed. In addition to presenting this work, the paper highlights some design criteria that any IPv6 stack targeting lowpower wireless networks should consider.