This document performs an analysis of the security and privacy implications of different types of "numeric identifiers" used in IETF protocols, and tries to categorize them based on their interoperability requirements and the assoiated failure severity when such requirements are not met. It describes a number of algorithms that have been employed in real implementations to meet such requirements and analyzes their security and privacy properties. Additionally, it provides advice on possible algorithms that could be employed to satisfy the interoperability requirements of each identifier type, while minimizing the security and privacy implications, thus providing guidance to protocol designers and protocol implementers. Finally, it provides recommendations for future protocol specifications regarding the specification of the aforementioned numeric identifiers.