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.