Abstract: A tunnel broker (4) configures a first node (2), which supports first and second communications protocols, e.g. IPv4 and IPv6, to communicate with a second node which supports IPv6, over an IPv4 network. The tunnel broker (4) assigns a unique IPv6 address (10) to the node, generated using a combination of the IPv4 address (11) of the node and a counter value (13). The counter is incremented for each user sharing an IPv4 address (11), so that each user sharing an IPv4 address (11) is allocated a unique IPv6 address (10). The tunnel broker service is restricted to users who have created an account. An account password is sent to the user's e-mail address, ensuring that a person giving a false address cannot gain access. A user cannot create further accounts using the same e-mail address. The number of tunnels that can be configured using each account is limited. These measures act to prevent an individual configuring a large numbers of tunnels (1) in a denial of service attack.