Some additional information on the map in today’s comic:
The labeled websites (“Google”, “Flickr”, “Slashdot”, etc) are just based on the location of the particular server I got when I pinged the site from my home machine on the day I was working on the map. Each site there has many servers and mirrors, and the IP of each may change from day to day. But their location is actually based on both the first and second quad — although the map is drawn in the style of a geographic map, with wobbly borders, it’s laid atop a regular grid which can be extended fractally downward. When I was putting down the location of a server for a particular website, I worked out where within the grid it should fall. This isn’t really for the purpose of telling you anything useful about that website, but rather to suggest that IPs can be placed as unique points on this map. Everything here was done by hand across a couple large sheets of paper, and it’s not trying to be completely accurate in all the details (although I did my best). It’s more of a survey than a practical road map.
The fractal behind the comic is a mapping of the integers to a plane (it could be extended, I assume, to the real numbers). I came up with it in 1999 when I was trying to figure out a way to graphically show the contents of long blocks of computer memory, which is basically linear. I wanted a mapping that would show strings of consecutive addresses as contiguous, compact blobs, and that would work on all scales. I played with and coded a couple mappings, but this was the only one I found that really fit that qualification.
Edit: The fractal is the Hilbert Curve, discovered in 1887. I had done some poking around but had never seen it before. Thank you, folks here and on IRC. It cannot be extended to the reals, but rather to the binary fractions (a subset of the rationals). This lets you get arbitrarially close to any number you want (and cover all the IEEE floating points).
I never come up with an algorithm to do the mapping — that is, a function that would take an integer and return the pair of coordinates. Years later, several months ago, I remembered the mapping and showed it to my cousin, who looked at it for a while and worked out a Scheme algorithm to do the mapping both ways.
Edit: I will be posting a version of the algorithm ASAP.
However, I also posed the problem as a puzzle to the forums:
I posted a sample of the mapping mangled into an integer-to-integer function and asked people to come up with the next set of numbers. The mangling was in part to make it less obvious what they were playing with and in part to encourage a different way of thinking about the problem. I informally offered two xkcd t-shirts as prizes for whoever provided the first correct answer and the most elegant algorithm. BaldAdonis very quickly took apart the problem (very impressively) and will — once I’m finished with this batch of shipping — get at least one t-shirt (depending on whether someone else finds an algorithm to do the mapping more elegantly). The other shirt is still up in the air. I’ll leave it open for the next week or two, and if you want to send me a piece of elegant and easily-executed code that does this mapping — taking in a number and returning the coordinates (bonus points for the reverse operation) — you’ll have a shot at winning a free xkcd t-shirt of your choice. (Note: for the next few days I’ll be working on t-shirt orders and then I have a wedding to go to, so I may not be able to check over your code until next this week.)
Suggestions for future work: When I was constructing the map, I also did some ping surveys of the various /8 networks to see how densely populated with active hosts each one was. The density data didn’t make it into the simpler final map, but that’s just one of many data sets one could slap up on a map like this to visualize the ‘net in an interesting way.