In order to carry out actions such as setting up a call from an entity in the address realm of one middlebox to an entity in the address realm of another middlebox, then a middlebox control node such as a call server is used. Previously, the middlebox control node has needed to have pre-configured information about all the middleboxes and which address realms they are associated with. The present invention provides one or more middlebox-identity-providing nodes which are separate from the middlebox control node, and which are more directly connected to the end users of the service than the middlebox control node. This provides greater flexibility in network design and removes the need for middlebox information to be pre-configured at the middlebox control node. Instead, this information is sent to the middlebox control node, as part of signalling messages, from middlebox-identity-providing nodes.