The NAT 6-4 translator maintains state and, for enabled interfaces, inspects the incoming IPv6 packets that match the configured filter (the filter specifies the IPv6 prefix) to determine whether the packet requires translation between IP versions.

Inspecting packets involves performing a lookup against configured IPv6 prefixes, deriving IPv4 addresses from IPv4 addresses, and performing lookups of IPv4 addresses against a table of converted addresses for the reverse flow. Translation implies changes to the Ethernet frame, the IP header, and updates to the TCP, UCP, and ICMP checksums. In brief, NAT 6-4 is highly invasive.