--- title: "What Is VAST? Video Ad Serving Template Explained" description: "VAST is the standard that lets a video player and an ad server exchange ads. See what a VAST tag contains and how VAST relates to VMAP and VPAID." url: "https://www.rentanott.com/glossary/vast" markdown_url: "https://www.rentanott.com/glossary/vast.md" category: "Advertising" updated: "2026-10-05" --- # What is VAST? VAST (Video Ad Serving Template) is the industry standard, published by the IAB Tech Lab, for how an ad server describes a video ad to a player. A VAST response is an XML document that tells the player which media file to play and which tracking addresses to call. ## How VAST works The player, or a server-side stitching service, requests a VAST tag: a URL that points to the ad server. The server replies with XML describing one or more ads. The player picks a suitable media file, plays it and calls the tracking URLs at the specified moments. A response can also be a wrapper that points to another ad server's tag, which is how an ad passes through several intermediaries before reaching the player. ## What a VAST response contains - Media files: the ad video in several formats and bitrates - Duration, and whether and when the ad can be skipped - Impression URL: called when the ad starts - Tracking events: start, first quartile, midpoint, third quartile, complete, mute, pause - Click-through URL - Companion ads and verification scripts ## VAST, VMAP and VPAID - VAST: describes an ad - VMAP: describes a schedule of ad breaks within a piece of content, with a VAST tag for each - VPAID: an older standard for interactive ads that ran code in the player, now deprecated in favor of newer VAST features and separate standards for interactivity and measurement ## Versions VAST 2 and 3 remain widely used; VAST 4 added features for server-side insertion, higher-quality media files and standardized verification. A player and an ad source must support a common version, which is worth confirming before signing with an ad partner. ## VAST on RentAnOTT AVOD is included on every RentAnOTT plan, and the advanced advertising add-on ($169/month) adds deeper targeting, frequency control and ad reporting. Bring your ad partner's tag details to a demo to confirm compatibility. [See pricing](https://www.rentanott.com/pricing). ## Related terms - [Ad server](https://www.rentanott.com/glossary/ad-server) — An ad server is the system that decides which ad to show each time a player or stitching service asks for one, delivers the details of that ad and counts the result. For a publisher it is the control center of an advertising business. - [CSAI](https://www.rentanott.com/glossary/csai) — CSAI (client-side ad insertion) is a way of delivering video advertising in which the player on the viewer's device requests each ad from an ad server and plays it itself, pausing the program while the ad runs. - [SSAI](https://www.rentanott.com/glossary/ssai) — SSAI (server-side ad insertion) is a way of delivering video advertising in which the ads are stitched into the stream on the server before it reaches the viewer. The player receives one continuous stream containing both program and ads. - [Dynamic ad insertion](https://www.rentanott.com/glossary/dynamic-ad-insertion) — Dynamic ad insertion (DAI) is choosing which ads to show at the moment a viewer reaches an ad break, individually for that viewer, instead of permanently embedding the same ads in the video file for everyone. - [Programmatic advertising](https://www.rentanott.com/glossary/programmatic-advertising) — Programmatic advertising is the automated buying and selling of ad impressions through software, in place of orders negotiated by sales teams. Each ad opportunity is offered to buyers and sold, often by auction, in the fraction of a second before the ad plays. - [Pre-roll ad](https://www.rentanott.com/glossary/pre-roll) — A pre-roll ad is a video advertisement that plays before the content a viewer chose. It is the simplest ad position to implement, since it needs no markers inside the video. - [Mid-roll ad](https://www.rentanott.com/glossary/mid-roll) — A mid-roll ad is a video advertisement that plays in a break partway through the content, in the manner of a television commercial break. It is the main ad position for long-form and live video. ## Further reading - [Add-ons](https://www.rentanott.com/features#addons) - [Content monetization platforms compared](https://www.rentanott.com/blogs/content-monetization-platforms) - [OTT revenue calculator](https://www.rentanott.com/tools/ott-revenue-calculator) ## Frequently asked questions ### What does VAST stand for? VAST stands for Video Ad Serving Template. ### What is a VAST tag? A URL that returns a VAST response when requested. Ad networks give publishers a tag to place in their player or ad stitching configuration. ### What does an empty VAST response mean? The ad server had no ad to return for that request. It counts as an unfilled opportunity and lowers fill rate. All 163 terms: [Streaming and OTT glossary](https://www.rentanott.com/glossary). ## Book a demo Tell RentAnOTT what you are launching. The team walks through the platform and maps the path to go-live around your catalog, markets and revenue model. Book a demo: https://www.rentanott.com/contact ## More from RentAnOTT - [Home](https://www.rentanott.com/index.md) — What RentAnOTT is, what every plan includes, and how launch works - [Features](https://www.rentanott.com/features.md) — Every core feature and optional add-on - [Pricing](https://www.rentanott.com/pricing.md) — Platform tiers, apps, add-ons, usage rates and example bills in INR and USD - [Free tools](https://www.rentanott.com/tools.md) — OTT app builder and calculators for streaming revenue, bandwidth, bitrate and pricing - [Insights](https://www.rentanott.com/blogs.md) — All streaming business articles - [Glossary](https://www.rentanott.com/glossary.md) — Definitions of streaming and OTT terms - [Book a demo](https://www.rentanott.com/contact.md) — Talk to RentAnOTT about your launch Full site in one file: https://www.rentanott.com/llms-full.txt