[keeganjw]

I still curse the IPv6 designers for not making it backward compatible with IPv4. IPv6 is definitely better designed but the lack of backwards compatibility makes moving to it an absolute bear. I know the designers thought the transition would only take a couple of years but almost 30 years later... here we are.

[orangeboats]

I challenge you to send a packet from 256.0.0.0 to any current IPv4 host.

"But IPv4 only ranges from 0.0.0.0 to 255.255.255.255"? Then congratulations on getting the point - it's pretty much guaranteed that any IPvNext address will have more bits than IPv4, therefore any IPvNext protocol is pretty much guaranteed to be backwards incompatible with IPv4.

IPv4 hosts simply cannot accept a packet nor send back a reply to IPvNext hosts, unless you rely on a middleman that does the translation between the two worlds.

[orangeboats]

To really drive the point home, representing an IPv6 address in the same way as an IPv4 address (4 dot-separated numbers) would make it look like this:

    536939960.3187088857.1487198345.2501756647
    
    (this is 2001:db8:bdf7:1dd9:58a4:d889:951d:c6e7)

You can't even squeeze that into a `sockaddr_in`.

[stonogo]

The entirety of ipv4 could have been embedded in ipv6 from the start. Instead the standards were written with no way for the two networks to interconnect, embedding ipv4 addresses into the link-local space within ipv6. This was a ridiculous decision, one of many that disincentivized adoption. ipv6 should have been designed with ease of transition from the beginning, instead of requiring dual-stack with a flag day that may never come.

[BingSwenSun]

OK. Now there is a compatible "next generation IPv4", nick name IPswen. For details, you may refer to this tweet?

https://twitter.com/BingSwenSun/status/1738513794933182671

What's notable is that IPswen is bidirectionally compatible with IPv4, meaning the two can interwork when the addresses are limited in the "Base Address Space" (the level 0 subspace of IPswen), much like the back and forward compatibility between color TV vs. black-and-white TV, or monophonic broadcasting vs. stereo broadcasting.

IPswen is a relatively new idea and still under development. As the history of networking and communications is replete with failures and technological disasters, it may be quite interesting to see how far it will actually go...

[jiggawatts]

You’re asking for the impossible.

“Curse you for not magically squeezing more than 4 billion addresses into 32 bits! Curse you!”

[BingSwenSun]

Well, IPswen can do this trick in a relatively simple way, and easily "squeezes" the whole IPv4 address space into its shortest length "level 0" base address space. The secret is simple: yes, variable length encoding. You may refer to the above reply for details.

[commandersaki]

Though interoperability is possible:

https://web.archive.org/web/20021017164820/http://cr.yp.to/p...

[rubatuga]

Dual stack is better than backwards compatible, in some cases.

[bauruine]

It is backwards compatible. There are multiple ways to connect to an IPv4 host with an IPv6 only host. Many mobile networks are IPv6 only. If you want it the other way around. How should that work?

[commandersaki]

ipv4->ipv6 with anycast tunnelling.