The most recent RunAs Radio podcast interviews Sean Siler on IPv6, the eventual replacement for IPv4, the current Internet protocol address scheme. At the current rate, we will run out of IPv4 addresses in May 2010. Previous estimated dates for running out of IP addresses have come and gone, and we keep coming up with ways to postpone the date. Still, no one disputes that we’ll run out of IPv4 addresses some day.

IPv4 uses 32-bit addresses and IPv6 uses 128-bit addresses. The latter uses four times as many bits, but that represents 296 times as many addresses. Siler illustrates this by saying that if the IPv4 address space were as wide as an atomic nucleus, the IPv6 address space would be a light-month wide.

I checked Siler’s calculation. Thirty light-days is 9.8 x 296 femtometers, so the analogy is correct for a nucleus of diameter 9.8 fm. According to Wikipedia, atomic nuclei range from about 1.6 fm (hydrogen) to 15 fm (uranium), so there’s some element in between for which he’s right.

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