&gt; If you run a service on IPv4+, your router needs to support IPv4+, and your users need to support IPv4+. That&#x27;s it.<p>What does &quot;your users need to support IPv4+&quot; mean? What does it take to make a &quot;user&quot; support IPv4+?<p>&gt; Note a user with an IPv4+ address can connect to non-IPv4+-capable systems since he also has NAT.<p>Yeah, the same as with IPv6&#x2F;v4 dual stack.<p>&gt; NAT already has this problem. Broadcasting ICMP is an acceptable solution -- remember that IPv4+ for a client is an alternative to NAT.<p>No, it doesn&#x27;t, no, it&#x27;s not. ICMP error messages contain the first 8 bytes of the packet that couldn&#x27;t be delivered, which in the case of TCP and UDP contains the port numbers, which a TCP&#x2F;UDP NAT uses to NAT the ICMP messages. And broadasting is not an acceptable solution because delivery of ICMP messages happens to sockets, not to machines. If you deliver ICMP UDP port unreachable messages to all sockets in the respective IPv4+ address range where the port happens to match, that&#x27;s going to cause chaos.