IP addres block Info

[Dave Feustel <dfeustel_(_at_)_mindspring_(_dot_)_com>]

I've started using tcpdump and pf.
I want to block the entire iprange of 
doubleclick.net, but I can't figure out
how to get the info. I tried nslookup
but that gives only specific addresses.
Then I tried nslookup interactively, setting
the server to a doubleclick ns, but it refuses
the commands that I think would give the ip
block assigned to doubleclick. How do I
get the ip address blocks assigned to doubleclick,
comcast, attbi, etc?

Thanks,
Dave Feustel