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THE very thing that makes the Internet a robust system, capable of surviving
lots of random damage, also makes it more susceptible to an intelligent attacker
intent on bringing it down. According to a mathematical model published this
week, if Internet nodes were to start failing at random, 18 per cent could
disappear and most of the Internet would remain connected. But if an intelligent
attacker targeted the most important nodes, the network would quickly break into
isolated fragments and stop functioning.

There are two major classes of networks: exponential and scale-free. In an
exponential network, all the nodes have about the same number of connections; in
a scale-free network, most nodes have a few connections, but some nodes have a
lot. The Internet is a scale-free network: a few computers, such as those
providing its backbone, have a lot of connections.

To see how this structure affects robustness, Reka Albert and colleagues at
the University of Notre Dame in Indiana built computer models of both kinds of
networks. When nodes in exponential networks fail at random, the network
performance decays quickly because all the nodes are equally important. When
nodes in scale-free networks fail at random, a lot can go missing without
causing a problem, because most nodes aren鈥檛 very important.

When their model eliminated the most-connected nodes first, however, the
scale-free network quickly collapsed into fragments. The exponential network
wasn鈥檛 so susceptible to an attack: with no highly connected nodes, it had no
obvious weak points.

Bruce Schneier of Counterpane Internet Security in San Jose, California, says
the paper doesn鈥檛 mention that defenders can make intelligent choices too. 鈥淭hat
is, in a scale-free system, you know which nodes are critical and can spend
resources securing them.鈥�