Ad networks for websites work by running a background auction every time a page loads — advertisers compete to buy that ad space in real time, and the highest bidder's ad gets shown, usually in well under a second.
We already covered the basic idea in our guide on what ad networks for websites are. This post goes one level deeper: what's actually happening in the split second between a page loading and an ad appearing.
It used to work like a line, not an auction
Imagine a school cafeteria with one lunch line. The first kid in line gets first pick of food, even if the second kid in line was willing to pay more for it. That's roughly how early ad networks worked. It was called a waterfall.
A website would rank its ad networks in a fixed order: try network A first, and if A didn't have a buyer, try network B, then C, then D. Whichever network happened to be first in line often won the ad space, even if a network further down would have paid more for that exact same impression. Nobody was actually competing against each other they were just waiting their turn.
Then it became a real auction
Modern ad networks for websites mostly run on real-time bidding instead. Real-time bidding flips the line into an actual auction. Instead of asking networks one at a time, "do you want this?", the page asks all of them at once, "what's your best offer?", and whoever bids highest wins that specific impression.
This entire process of Ad Networks Win More Bids, called header bidding when it happens through multiple demand sources simultaneously, takes place in the time it takes a page to load. Here's what happens, in order:
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A visitor opens a page with an empty ad slot
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The page instantly sends that ad slot out to many advertisers at once, not just one network
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Each advertiser's system calculates a bid in real time, based on things like the visitor's rough location, device, and browsing context
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All the bids get compared side by side
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The highest bid wins, and that specific ad is shown, all before the page has fully finished loading
The whole exchange usually takes a few hundred milliseconds. It happens every single time a page loads, for every single visitor, which is why two people visiting the exact same page at the exact same time can be shown completely different ads at completely different prices.
Why does this matter for your revenue?
The waterfall version had a real weakness: a lower network could win an ad slot simply because it was asked first, even if a better-paying advertiser was sitting one step behind it in line, never given the chance to bid. That means real money was left unclaimed on every single ad slot, every single time.
Running ad networks for websites through real-time bidding fixes that weakness directly, because every advertiser gets a genuine chance to compete on every impression, not just the ones who happened to be asked first. That's the entire reason header bidding became the standard: it turns a fixed line into a fair auction, and fair auctions pay more.
FAQ
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What's the difference between a waterfall and real-time bidding?
A waterfall asks ad networks one at a time in a fixed order, so whoever is first often wins by default. Real-time bidding asks everyone at once and lets the highest bid actually win.
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How fast does the ad auction happen?
Usually a few hundred milliseconds, all while the page is still loading, so the visitor never notices any delay.
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Why do two people see different ads on the same page?
Because the auction runs fresh for every single visitor, factoring in things like their device and location, so the winning bid can be completely different from one visitor to the next.
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Is real-time bidding always better than a waterfall for a website?
In almost all cases yes, since it lets more advertisers genuinely compete for the same space instead of one network winning just for being first in line.
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