What Is an Ad Exchange? How the Auction Works

What is an ad exchange?

An ad exchange is a digital marketplace where advertisers and publishers buy and sell ad impressions one at a time, in real-time auctions. When a page or app has an ad slot to fill, the exchange sends a description of that impression to many buyers, collects their bids within a deadline, and the highest qualifying bid wins. Google’s own Ad Manager resources describe an exchange as “an online, auction-driven marketplace where ad impressions are bought and sold in real-time.”

The exchange is the middle of the programmatic chain. What is programmatic advertising? gives the overview, and DSP vs SSP covers the buying and selling platforms on either side.

How an ad exchange auction works

Each auction follows the same pattern, usually in well under a second:

1. An impression becomes available. A user opens a page or app screen, and the publisher’s ad server or SSP passes the opportunity to the exchange.
2. The exchange sends a bid request. It describes the slot, the site or app, the device, a rough location, any IDs the user’s consent allows, the price floor, any deals, and a deadline.
3. Buyers decide. According to Google’s page on how Authorized Buyers work with Ad Manager, a buyer’s bidder “analyzes the information, selects an advertiser from their base of clients” and returns a creative and a bid, or places an empty bid if it does not want the impression.
4. The auction runs. The exchange drops bids that are late, below the floor, or from blocked advertisers or categories, and ranks the rest.
5. The winner’s ad serves. The winner is notified, the impression is counted, and the advertiser is billed, usually on a CPM basis.

Google says the whole process “happens within around 100 milliseconds.” The exact deadline is set per request: in OpenRTB it is the tmax field, and Google’s Authorized Buyers guide says its deadline “typically ranges from 80 to 1000 ms” (checked on November 4, 2026).

Who are the buyers? Google lists three kinds of Authorized Buyers: ad networks, agency trading desks, and demand-side platforms. In other words, ad networks can be buyers on an exchange too.

Ad exchange vs ad network vs SSP

These three get mixed up because one company often runs more than one, but they do different jobs.

In practice the line between an SSP and an exchange has almost disappeared, because most SSPs run their own exchange. The difference with a network is still real: a network decides what you are paid and which ad shows, while on an exchange each buyer prices each impression. What is an ad network? covers networks in depth.

First-price vs second-price auctions

In a second-price auction, the winner pays just above the second-highest bid. In a first-price auction, the winner pays what it bid. The OpenRTB 2.6 specification has a field for this, at, where “1 = First Price, 2 = Second Price Plus”, with exchange-specific types allowed from 500 upward; if the field is left out, it defaults to 2, which shows how standard second price once was.

The market has moved to first price, and Google was the biggest step. In a September 5, 2019 post, Google said it was “transitioning to unified first price auctions for Google Ad Manager”, and that its tests showed a neutral to positive effect on publishers’ total revenue on average compared with second-price auctions.

What changes for you as a publisher:

• Floors matter more. In first price, a floor is simply the lowest price you accept; there is no second bid to lift the price.
• Buyers bid more carefully. Since they pay what they bid, buy-side tools try to bid only as much as needed to win, so competition between many buyers is what keeps prices up.
• Reports are easier to read. The winning bid is the clearing price, before fees.

Price floors, blocks and deals

An exchange is not a free-for-all. As a seller, you or your SSP control:

• Floors. OpenRTB carries a bidfloor, the “minimum bid for this impression expressed in CPM”. Floors set too high cut fill; floors set too low leave money behind. Set them by country and format. What is fill rate? explains the balance.
• Blocks. Advertiser domains, categories and creative types you do not want on your pages.
• Deals. Private auctions, preferred deals and programmatic guaranteed run through the same exchanges, identified by a deal ID. What is programmatic advertising? compares the deal types.

Examples of ad exchanges

These are well-known names described in their own words, not a ranking:

• Google Ad Exchange (AdX), sold to publishers as part of Google Ad Manager, which is also a publisher ad server. Authorized Buyers, which Google says include ad networks, agency trading desks and DSPs, bid into it.
• Magnite, which calls itself “the largest independent sell-side advertising company”.
• Index Exchange, which describes itself as “a global supply-side platform” and runs its own exchange.
• PubMatic, a sell-side platform listed on Nasdaq.

In mobile apps, exchanges usually reach you through mediation and in-app bidding rather than directly. In-app bidding vs waterfall and What is ad mediation? explain that path.

The Google ad tech case and what it means

Ad exchanges have also been at the center of a major US antitrust case. On April 17, 2025, the US Department of Justice announced that the federal court in the Eastern District of Virginia had found Google violated antitrust law by monopolizing open-web digital advertising markets. As the National Law Review’s summary of the remedies opinion explains, the court found monopolies in publisher ad servers and ad exchanges, tied together through Google’s ad server (DFP) and AdX.

In its remedies opinion, unsealed on September 16, 2026, the court declined to order Google to sell AdX. It instead ordered behavioral remedies for six years, including:

• no tying of Google’s publisher ad server to AdX;
• a ban on First Look, Last Look and Unified Pricing Rules for indirect transactions, so publishers can set different floors on different exchanges;
• integrations so Prebid can request real-time AdX bids for indirect open-web display inventory.

This summary was checked on November 4, 2026; the summary says the remedies take effect 60 days after the order is entered, and news reports say Google is appealing, so check the latest news before you change your setup because of them. For header bidding publishers, AdX demand through Prebid would mean one more major bidder in the same auction. What is header bidding? explains how that auction works.

Exchanges, sellers.json and supply chain checks

Because the same impression can be offered through several exchanges and resellers, buyers check who is selling. IAB Tech Lab’s sellers.json page says SSPs and exchanges are expected to publish a sellers.json file listing the sellers and intermediaries they work with, and that the SupplyChain object in each bid request lists every party that sold or resold it. Together with your ads.txt or app-ads.txt, that lets a buyer confirm the exchange is authorized to sell your inventory. Buyers also cut long or unclear paths, so selling through fewer, direct paths tends to help you. Invalid traffic is filtered at every step; Ad fraud and bot traffic covers the warning signs.

Where an offerwall fits

An exchange prices impressions. An offerwall does not sell impressions at all: it is a non-programmatic, pay-per-result format where advertisers agree a payout for a completed action, such as installing a game and reaching a level, and users choose offers to earn your site’s or app’s own currency. There is no bid request, no auction and no floor, and the result is confirmed by a server-to-server postback, which can also reverse it later. It sits beside your exchange demand as its own earn screen, and fits products with logged-in users and a points or coin system, not sites with only anonymous readers. Offerwall vs rewarded video compares it with an exchange-sold format.

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