A method of delegating responsibility for an IP address owned by a first IP network node to a second IP network node, at least a part of the IP address being derivable from a public key of a public/private key pair belonging to the first node. The method comprises notifying the first node of a public key of a public/private key pair belonging to the second node, at the first node, signing the public key of the second node with the private key of the first node to provide an authorisation certificate, and sending the authorisation certificate from the first node to the second node, wherein the authorisation certificate is subsequently included with messages relating to said IP address and signed with the private key of the second node, sent from the second node to receiving nodes, and is used by the receiving nodes to verify the second node's claim on the IP address.