A few weeks ago the global routing table reached its 300,000th route.  Below is a graph showing the exponential growth over the last 15 years.

Let’s do a little math:

Assuming all 4.3 billion IPv4 addresses are used (which isn’t quite true), each route represents approximately 4,294,967,296 / 300,000 = 14317 addresses. This is almost equivalent to a /18 (16,384 addresses). However, there are only 2^18 = 262,144 subnets of this size.

Why are there so many routes in the table?

Because there are BGP Administrators who advertise junk like this.  AT&T WorldNet Services is advertising over 1100 prefixes; most of them are /24s.  Due to their lack of summarization, this one group of routers is responsible for almost .5% of the fluctuation in global routing tables during any given week.  That’s really bad.

Poke around here for some more info on BGP and the global routing table.