How does the bidding system work in an adnetwork

I have a situation here..
Suppose an ad network is having 100 website with 100K impressions for a specific delivery rule.
Now if an advertiser places his ad and bids an amount and a budget of only 10K impresions then how are his 10K impressions distributed among the 100 websites with a total of 100K impressions.
Do they get randomly distributed among all the websites. How does it happen in an ad network.
regards.

See original post by UrAdGinny