Title text: I think the IETF hit the right balance with the 128 bits thing. We can fit MAC addresses in a /64 subnet, and the nanobots will only be able to devour half the planet.
Megan and Ponytail are in orbit while nanobots are devouring the Earth in a swarm. This is a take on the "Grey goo" scenario in which self-replicating nanobots destroy the earth while creating more and more of themselves non-stop.
Unusually, however, the nanobots stop after devouring 40% of the planet. This is because each individual nanobot must apparently have a distinct IPv6 address, and by running out of possible addresses the nanobots cannot produce any more of themselves. With the tiny size of each nanobot, the total volume they can contain is only 40% of the Earth's mass, and they can no longer continue their consumption of the planet.
IPv6 supports approximately 3.4×1038 addresses, while the Earth's mass is around 5.972×1024 kg. Assuming "a few cubic microns" is the minimum of 2 µm3 (according to 1070: Words for Small Sets), the nanobots would have a density of 4 g/cm3, a bit less dense than the Earth.
1998 is when the IPv6 Specification (RFC 2460) was published and IETF is the Internet Engineering Task Force.
Note that an April Fools' joke for IPv9 exists and would have guaranteed Earth's doom in this comic's scenario.
- [Megan and commander are on a space station.]
- Megan: Commander! Come quick! It's the nanobots—they've STOPPED!
- Megan: They devoured 40% of the Earth, and then just... quit! They're just sitting there! Why?!
- Ponytail: It's a mystery. ...unless... What's the volume of each nanobot?
- Megan: A few cubic microns. Why?
- Ponytail: I think the year 1998 just bought us some time.
- [Earth's surface, covered in mountains of nanobots.]
- In the swarm:
- Nanobot: What do you mean, "Run out of addresses?"
- Other Nanobot: Look, we should've migrated away from IPv6 AGES ago...
- On the website, the space above the comic says "xkcd.com now has IPv6 connectivity. If you can't reach it, you or your ISP have misconfigured equipment. Sadly, I now have no way to tell you."
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