IPv6

Regarding IPv4-compatible IPv6

I hear this desire a lot from armchair designers, but I've yet to have one enumerate an actual way for IPv6 to be retro-compatible with IPv4 that isn't present in IPv6 already.

IPv4 addresses are a subset of IPv6 addresses - if you have an IPv4 address, then it also exists in (at least) five different forms for different (mostly failed) compatibility mechanisms in IPv6:

• As an "IPv4-Compatible IPv6 Address" in ::/96 - e.g. 192.0.2.1 is also ::c000:21. The transition mechanisms that wanted to use this form of addressing (e.g. the original form of SIIT) never reached deployable state, and thus this form is now deprecated.
• As an "IPv4-Mapped IPv6 Address" in ::ffff:0:0/96 - e.g. 192.0.2.1 is also ::ffff:c000:21. This is still in use, so that applications can pretend that IPv4 does not exist, and use these to refer to IPv4 addresses (when talking to the network stack, and in some protocols).
• In 64:ff9b::/96 for NAT64 and 464XLAT purposes - e.g. 192.0.2.1 is also 64:ff9b::c000:21. This allows you to communicate over an IPv6-only network with a central IPv4 host that does NAT for you, getting you the CGNAT experience when talking to IPv4 hosts; this is deployed in (at least) T-Mobile USA and EE UK mobile networks.
• Lots of times in 64:ff9b:1::/48 (depending on provider) to allow you to have multiple NAT64 or 464XLAT deployments addressed differently.
• As a 6to4 prefix in 2002::/16, for example 192.0.2.1 gets you control of all of 2002:c000:21::/48. This lets you route IPv6 as an overlay on your existing IPv4 network, but is not popular because properly deployed IPv4 will always be at least as fast as or faster than IPv6 carried over an IPv4 network.
• On top of that, SIIT got reworked into SIIT-DC, which allows a pure IPv6 network to choose a prefix for IPv4 use and do stateless NAT instead of NAT64. And, of course, you can do NAT64 or 464XLAT in a private prefix.

Each of these mechanisms has its own set of problems as compared to running dual stack networks; 464XLAT and NAT64 are only happening now because in the mobile world, operators are beginning to experience pain from running NAT'd IPv4, and reducing the need for NAT via IPv6 saves actual dollars. And, if nothing else, pure IPv4 has the advantage of zero change needed - any alternative IPv6 proposal needs to cope with IPv4-only hosts that refuse any form of change to support IPv6, otherwise you face the same problems as SIIT, NAT64, and 6to4 do.

Neighbor Discovery

A kinda-replacement for ARP, DHCP and mDNS carried over ICMPv6.