NPTv6 is a stateless, transport-agnostic IPv6-to-IPv6 Network Prefix Translation function that provides the address-independence benefit associated with IPv4-to-IPv4 NAT without some of that mechanism's drawbacks. NPTv6 is intended to be deployed in environments where a host might not know its "external" network prefix(es). In such an environment, a NPTv6 device must also translate network prefixes present in in-bound and out-bound ICMPv6 error message payloads. This document describes the required processing of ICMPv6 error messages.