In computer networking terminology, a tunnel broker is the phrase used to describe a service that provides a network tunnel. The Tunnel Broker model is based on dedicated servers called Tunnel Brokers that automatically manage tunnel requests coming from users.
The most common usage of the phrase is in reference to IPv6, where a tunnel broker provides IPv6 tunnels to Web sites and users. The Network Working Group defines IPv6 Tunnel Broker in RFC 3053. The IPv6 global Internet uses a lot of tunnels over the existing IPv4 infrastructure. The tunnels are difficult to configure and maintain and too complex for the isolated end user, so the concept of the tunnel broker was presented to help early IPv6 adopters to hook up to an existing IPv6 network.
See tunneling and RFC 3053: IPv6 Tunnel Broker.
See also “Understanding IPv6” in the Did You Know…? section of Webopedia.