This document creates a new IANA registry (called the "Post-Stack First Nibble" registry) for the first nibble (4-bit field) immediately following an MPLS label stack. Furthermore, this document presents some requirements for registering new values and making the processing of MPLS packets easier and more robust. The relationship between the IANA "Post-Stack First Nibble" registry and the "IP Version Numbers" registry (RFC 2780) is described in this document. This document updates RFC 4928 by deprecating the heuristic method for identifying the type of packet encapsulated in MPLS.