An ad network is an intermediary that aggregates ad inventory from many publishers and sells it to advertisers, acting as a traffic broker between the supply and demand sides of the market. Ad networks were one of the earliest solutions to a structural problem in digital advertising: individual publishers struggled to sell all their inventory directly, while advertisers couldn't negotiate with thousands of sites one by one. The network bundles inventory — often grouped by audience segment, vertical, geography, or ad format — and resells it as packages.
 

The defining characteristic of an ad network is aggregation and repackaging. The network buys or represents publisher inventory in bulk, then offers it to advertisers, typically taking a margin on the difference between what it pays publishers and what it charges advertisers. This contrasts with an ad exchange, which trades inventory on a transparent, per-impression basis through real-time auctions without bundling.
 

For advertisers, ad networks offer simplicity and reach. Rather than managing dozens of publisher relationships, a buyer accesses a large pool of inventory through a single point of contact, often with targeting options layered on top — geo, device, demographic, behavioral, and contextual. Many networks also provide managed-service support, handling campaign setup and optimization for advertisers who lack in-house programmatic expertise.
 

For publishers, networks provide demand and fill. A publisher that can't sell all its impressions directly can route the remnant to one or more networks to recover revenue that would otherwise go unmonetized. Publishers often work with several networks simultaneously, historically chaining them in a "waterfall" so unsold inventory cascades from one to the next until it's filled.
 

Ad networks vary widely in quality and specialization. Some focus on premium brand-safe inventory; others on performance, push, native, or specific verticals like crypto or gaming. The trade-off advertisers accept with networks is reduced transparency: because inventory is bundled, buyers may have less visibility into exactly which sites their ads appear on compared to a transparent exchange environment.
 

The rise of programmatic, RTB, and header bidding has reshaped the network model. Many networks evolved into or integrated with exchanges and SSPs to offer real-time, transparent, per-impression buying. Today the lines blur — a single platform may operate as a network, exchange, and SSP at once. Still, the core function endures: connecting fragmented supply with scaled demand, and giving both sides a practical way to transact efficiently.