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Mar 27 2009   2:27PM GMT

Overheard – IPv6 not backwards compatible with IPv4?

Margaret Rouse Margaret Rouse Profile: Margaret Rouse

The Internet engineering community says its biggest mistake in developing IPv6 – a long-anticipated upgrade to the Internet’s main communications protocol – is that it lacks backwards compatibility with the existing Internet Protocol, known as IPv4.

Carolyn Duffy Marsan, Biggest mistake for IPv6: It’s not backwards compatible, developers admit

I just finished reading Carolyn Marsan’s piece, Google: IPv6 is easy, not expensive.  Google, you see, has moved to IPv6IPv6 operates in much the same way as IPv4, but with one very important distinction — IPv6 assigns IP addresses of 128 bits instead of IPv4’s 32 bits and that really increases the total number of possible Internet addresses.

Carol reports that Google engineers worked on the IPv6 effort as a 20% project – meaning it was in addition to their regular work.  So far, the U.S. has been lagging behind other countries with IPv6 — mostly because we can — but now that Google has come on board, that may hurry things along.

What really caught my interest,  was a sidebar that linked to an article where Carol did a bang-up job explaining the REAL issue that is holding up IPv6.  Simply put, the developers mis-judged how adoption would really occur.  They may have shot themselves in the foot by not making IPv6 backwards-compatible with IPv4, but It’s not their fault — it would have been cost-prohibitive, complicated and illogical in some ways.

(Explaining it to myself) It would be like your television station broadcasting in both analog and digital for awhile and then gradually fading out analog broadcasts as people replaced their old analog sets with new digital ones.  It seemed like a logical, practical plan.

The IETF developers designed IPv6 to run in a dual stack.  That means that IPv4 and IPv6 would run side by side for awhile and then IPv4 would gradually be faded out.

They didn’t foresee a scenario where IPv4 devices would stick around for years, some vendors wouldn’t bother upgrading their products to be IPv6-compliant and some administrators would just shut off the IPv6 part of the dual stack in an effort to keep things simple.

Since IPv4 isn’t fading away as the engineers had thought, they are going back to the drawing board to help IPv6 addresses be understood by IPv4 devices.

Carol says the transition mechanisms include:

* Dual-Stack Lite, a technique developed by Comcast that allows for incremental deployment of IPv6. With Dual-Stack Lite, a carrier would give new customers special home gateways that take IPv4 packets from their legacy PCs and printers and ship them over an IPv6 tunnel to a carrier-grade network address translator (NAT).

* NAT64, a mechanism for translating IPv6 packets into IPv4 packets and vice versa. A related tool, dubbed DNS64, allows an IPv6-only device to call up an IPv4-only name server. These two tools would allow an IPv6 device to communicate with IPv4-only devices and content.

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