A tunnel broker is a service which provides a network tunnel. These tunnels can provide encapsulated connectivity over existing infrastructure to a new infrastructure.
There are a variety of tunnel brokers, though most commonly the term is used to refer to an IPv6 tunnel broker. These commonly provide IPv6 tunnels to endusers/endsites using either manual, scripted or automatic configuration. In general tunnel brokers offer so-called 'protocol 41' or proto-41 tunnels. These are tunnels where IPv6 is tunneled directly inside IPv4 by having the protocol field set to '41' (IPv6) in the IPv4 packet.