IP address discovery

A method and apparatus each facilitate communication between users of a SIP-based network and SIP users external to that network. Each user of the SIP-based network possesses a SIP URI comprising a host name part belonging to the network and a user name part. The external users have access to a peer-to-peer network to resolve SIP URIs into contact IP addresses. The method comprises publishing into said peer-to-peer network a mapping between an identifier of a gateway to said SIP-based network and an IP address of the gateway. The identifier corresponds to or is derivable from said host name part. Upon receipt of a query at the gateway in respect of a URI associated with one of said users of said SIP-based network, the URI is resolved into a contact IP address. The contact IP address is returned to a node from which the query originated.