I will also point out that many of the recent "smurf" attacks and
similar problems people are having on the net would be gone if people
would just carefully filter internal/external addresses on their
border machines, that is, prevent packets claiming to be from "inside"
networks from coming in from the "outside", and prevent packets
claiming to be from "outside" networks from going out from the
"inside". The latter will stop your network from *ever* being the
source of a wide variety of packet forgery attacks, and is necessary
to being a good network citizen. The former will stop your network
from being the subject of a wide variety fo packet forgery attacks,
and is necessary to make your customers even remotely safe on the net.
That's great if you're a downstream provider with no transit customers.
However, when you become a transit provider, it becomes much more difficult
to determine inside vs. outside, since you're more in the middle between
two "outsides" that pass traffic through you.