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How Ad Servers Work

Written by Roy
6 min read
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1. What is an ad server?
2. How does an ad server work?
3. What are ad servers used for?
4. Benefits of ad servers
5. Ad servers vs. ad networks
6. Examples of ad servers
7. Conclusion

What is an ad server?

An ad server is a platform used to manage and deliver online ads on publisher's websites and apps. Ad servers play an important role in digital advertising as it automates the process of ad placement across websites, while ensuring that ads are served to the right user at the right time.

There are two types of ad servers: first-party ad servers and third-party ad servers. Let's break that down first.

First-party ad servers

First-party ad servers are also called publisher ad servers and they are one of the first to be called when a user opens a web page. Publisher use them to manage and deliver ads directly on their own websites or apps.

A first-party ad server has features to optimize ad inventory, control what type of ads are displayed and ensure a quality ad experience by using ad targeting and frequency capping.

Third-party ad servers

Third-party ad servers are used by advertisers and therefore also called advertiser ad servers. Their goal is to deliver ads across multiple publishers, while tracking performance and optimizing campaigns in an automated way.

Ad servers for advertisers have features that are different from those you'll find in a publisher ad server.

Here's an overview of the main differences between first-party ad servers and third-party ad servers.

Difference between first party ad servers and third party ad servers.
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How does an ad server work?

Ad servers, like AdGlare, work through a series of steps to deliver the right ads to the user. This generally happens in less than a few milliseconds.

Let's see how that works in more detail for a publisher ad server:

  1. The ad request. When a user visits a web page, the browser sends loads the ad tag and sends a request to the ad server. The ad tag is a piece of JavaScript code with information about the position and name of the ad placement, so that the ad server knows which set of ads to return.
  2. Making a list of campaigns. The ad server now makes a list of all campaigns assigned to the ad placement. It takes the campaign's flight dates into account.
  3. Eligibility check. Then, the ad server filters campaigns that are not eligible, for example those that are meant to serve only on mobile, or to users in certain geographical locations.
  4. Picking the winner. Now that a list of eligible ads has been made, the ad server picks an ad to be served. This can be randomly, based on weights, or even on the amount of money the advertiser wants to pay in order to display his ad.
  5. Returning HTML/JS code. The ad server responds to the request by returning the code that makes the actual ad. The browser will then execute that code in order to display the ad.
  6. Impression and click counting. Now that the ad is displayed on the page, the ad server counts an impression. The ad code will trigger a call to the ad server when the user clicks on the ad, so that the ad server can log the number of clicks and calculate performance metrics like CTR.
  7. Performance tracking. Internally, the ad server keeps a database with statistical data about event counts and presents the results to the publisher or advertiser in the user interface. These reports are used to invoice the advertiser.
Infographic how ad servers work
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But what if a third-party ad server is involved?

When an advertiser ad server is involved, the workflow includes additional steps between step #5 and #6. Instead of returning the code that makes up the ad, the publisher's ad server invokes the advertiser's ad server via a second ad tag. The advertiser's ad tag is then executed by the browser and the third-party ad server responds with the actual HTML/JS code that creates the ad.

Including a third-party ad server allows for both parties (publishers and advertisers) to track key metrics at the same time. Advertisers can then compare report numbers with the ones provided by publishers, before paying the publisher for the campaign. While comparing reports, ad discrepancies are usally a source of debate.

What are ad servers used for?

Ad servers have various roles and important features, for both parties in the digital advertising ecosystem. Depending on whether you're a publisher or an advertiser, here are some common reasons why ad servers are used:

  • Ad management: ad platforms allow people to manage ad campaigns, including targeting rules, flight schedules and the pages on which they can appear.
  • Creative delivery: ad servers store the actual creative on the web server, so they act as a content delivery server.
  • Detailed reports: event data like impressions, clicks and conversions give insights to key metrics like CTR and conversion rates.
  • A/B testing: ad servers are great is testing different versions of an ad and provide realtime metric data to see which one is the winner.
  • Budget management: instead of serving all impressions in the beginning of a campaign, ad servers allow for even pacing throughout the duration of the campaign.

Benefits of ad servers

Ad servers are a key element in the online advertisement ecosystem. Although, in theory, we could do without, ad servers have benefits for both publishers and advertisers.

Ad servers help publishers by managing ad inventory (ad spots), while increasing ad revenue by serving the most relevant or highest paying ads first. Publishers have full control over which ad is displayed when and where.

For advertisers, ad servers are important to optimize campaigns by providing reports with key metrics like CTR and conversion rates. If a campaign is not doing well, advertisers can change the creatives on the fly without the need to contact the publisher.

Ad servers vs. ad networks

Too often we get the question if AdGlare provide ads to publishers. Let me therefore briefly explain the difference between ad servers and ad networks. While both are important components in the ad tech industry, they serve a completely different purpose and have different functions.

An ad server stores creatives, select the best ad and provides reports on ad performance. It focuses on the technical aspect of getting the ad in front of the user. Ad servers are used by publishers to serve campaigns made from direct deals with advertisers.

Ad networks, on the other hand, are marketplaces that act as a middleman between advertisers and publishers. By aggregating ad inventory from multiple publishers, they offer a variety of ad placements to advertisers. Ad networks simplify the process for advertisers to find inventory, while at the same time offering a one-stop solution to publishers to monetize their content.

Ad networks generally use ad servers to manage inventory and serve campaigns.

Examples of ad servers

Are you looking for an ad server? In this section, I will outline some of the most prominent players in the ad tech industry. Here are some ad server examples:

  • AdGlare AdServer

    AdGlare Ad Server is a simple yet robust ad server for both publishers as well as advertisers. It has features like geotargeting, dayparting and real-time reporting. Known for its reliable infrastructure and user-friendly interface, AdGlare is ideal for both publishers and advertisers looking to serve, manage and optimize their ad campaigns.
  • Google Ad Manager

    Google Ad Manager is a comprehensive ad serving platform for publishers. It integrates with Google's advertising ecosystem and offers advanced targeting, detailed analytics and multiple ad formats.
  • Sizmek by Amazon

    Sizmek provides advertisers with a powerful platform for creating, distributing and optimizing ad campaigns. It offers sophisticated targeting options and comprehensive analytics.
  • Smart AdServer

    Located in France, Smart AdServer offers a full-featured ad serving solution with advanced targeting, real-time bidding and detailed reporting. It was designed to help both publishers and advertisers achieve their advertising goals.
  • OpenX

    OpenX is a popular publisher ad server known for its real-time bidding capabilities and extensive ad exchange network. It provides publishers with tools to maximize their ad revenue mainly through programmatic advertising.

List of ad servers

There's also an interactive list of ad servers, where you can compare features, requirements, costs and more.


An ad server is a platform that manages and delivers online ads, making sure they reach the right user at the right time. There are two types: first-party ad servers, used by publishers to manage ads on their properties (website, app) and third-party ad servers used by advertisers to distribute ads across multiple websites while tracking performance.

Ad servers handle tasks like campaign eligibility checking, ad selection, creative optimization and performance tracking. They play an important role in the digital advertising ecosystem ever since the first ad campaign was launched.

Examples of ad servers include AdGlare, Google Ad Manager, Sizmek, Smart AdServer and OpenX.

About AdGlare

As an established ad server, AdGlare has over 11 years of experience in managing, serving and optimizing ads. Reach out to see how AdGlare can help you achieving your advertising goals. Or sign up for a free 14-day trial to take a quick look inside.

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