1. A method of operating a computer system comprising a plurality of computer nodes, at least one service executing on at least one of the computer nodes in a first subnet, an availability monitor that maintains the availability of the service on at least one of the computer nodes and may relocate the service from one of the computer nodes in the first subnet to another computer node in a second subnet, at least one client computer executing at least one application program adapted to access the service, and a domain name service (DNS) that provides at least one network address at which the service is accessible to the client computer, the client computer comprising a local store that has at least one entry storing, from the DNS, the network address at which the service is accessible, the method comprising: in response to the service being relocated by the monitor from a node in the first subnet to a node in the second subnet so that the service is no longer accessible at the network address stored in the local store for the client computer, providing, through an event services agent, a notification to the client computer before DNS updates the network address in the local store, the notification indicating that the service is no longer accessible via the network address stored in the local store for the client computer.