Basics
Open web
The open web is the part of the internet outside the big closed platforms, such as independent websites, apps and streaming services, where ads are bought through shared, interoperable ad tech.
"Open web" is the industry's name for everything that is not a walled garden. Google Search, YouTube, Meta, Amazon and TikTok run their own closed ad systems; the open web is the millions of news sites, blogs, apps and CTV services that sell ads through independent SSPs, DSPs, header bidding and standards such as OpenRTB.
The open web is defined by interoperability: any buyer can, in principle, reach any seller through common protocols, and third-party measurement can check the results. That openness is its strength and its weakness. It supports independent journalism and competition, but its long supply chains create room for the ad tech tax, made-for-advertising sites and ad fraud.
The open web's share of ad spending has shrunk as walled gardens grew. Companies such as The Trade Desk position themselves as champions of the open internet. Note that "open web" is not a technical boundary: Google also operates major open-web ad tech (Ad Manager, AdX), which was central to the United States v. Google case.
Think of it like this
The open web is like a city's high street of independent shops that all accept the same payment cards, while walled gardens are giant malls with their own currency.
An example
An airline buying ads across 5,000 news and travel sites via a DSP is buying on the open web; buying the same audience on Instagram is buying inside a walled garden.
Related terms
Walled garden
A walled garden is a large platform, such as Google, Meta, Amazon or TikTok, that controls its own users, data, ad sales and measurement inside a closed ecosystem.
Programmatic advertising
Programmatic advertising is buying and selling ad space through automated software, usually one impression at a time, instead of through manual negotiations, phone calls and paper contracts.
Header bidding
Header bidding is a technique where a publisher offers each ad impression to many SSPs and exchanges simultaneously before calling its ad server, so all buyers compete at once.
Supply path optimization (SPO)
Supply path optimization (SPO) is a buyer's practice of choosing the most direct, cheapest and most trustworthy routes to the same inventory, and cutting unnecessary resellers and duplicate paths.
Ad tech tax
The ad tech tax is the combined share of an advertiser's spend taken by intermediaries, such as agencies, DSPs, data providers, verification vendors and SSPs, before the rest reaches the publisher.