CPM ads from big companies

I see some large websites that have AT&T ads and AAA ads and other ads from big name companies.  I figure these bigger advertisers will pay more than Google ads.  How do these sites get these big name ads on their site?  Not that my website is anywhere in that league,  but just curious.  Are these CPC or CPM programs?  Or did the website owner contact that advertiser themselves?

You can host ads from big companies when you are on big networks like Casale and Valueclick or affiliate programs like CJ (I think they have some Sony products too)

In either case I don't think you can have those campaigns with less than a few million impressions per month ;)