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The Internet is made up of independently managed but interconnected networks called autonomous systems or ASs. Lower-level service providers need to use the routes across higher-level service providers. Many national service providers also need to exchange routes with one another. Peering refers to a relationship between two service providers that agree to exchange traffic and routing policies, usually across a direct link that the two service providers establish. An analogy is the way that states allow the free exchange of traffic across their highway systems.

Peering agreements may take place between local ISPs or ISPs that provide the major backbone networks of the Internet.

Peering also involves the exchange of routing information among networks. The information is in aggregated form and describes only the prefixes. See "Route Aggregation" and "Routing Registries."