Independent Researcher This document specifies Unified Transition Overlay (UTO), a stateless encapsulation-plus-translation mechanism that enables bidirectional communication between IPv4-only and IPv6-only hosts across a transit network whose forwarding plane uses the opposite IP version. UTO carries the native source and destination addresses of the communicating hosts in a compact, variable-length overlay header. The original packet is tunneled between UTO-Gateways (UGWs) across the transit network and is converted to the destination's address family by a single stateless header translation performed only at the egress UGW. UTO requires no DNS64, no stateful per-flow translation in the common IPv4-to-IPv6 direction, and no changes to backbone routers or end-host stacks. The transit network forwards only pure IPv4 or pure IPv6 packets.