An address system for determining an endpoint address in a network based on an address query, the address system being configured to: receive an address query from a device of the network, said address query being formed in dependence on a type of a client instance executed at the device, said address query being for requesting a network address of an endpoint (110 1) of the network; store relationships which map address queries to network addresses, wherein the stored relationships distinguish between address queries formed in dependence on different types of client instance; use the stored relationships to determine an endpoint address in the network based on the address query such that the determined endpoint address is dependent upon the type of the client instance executed at the device; and send an answer to the device in response to the address query, said answer including the determined endpoint address; the address system characterized in that the type of the client instance upon which the address query and endpoint address depend is given by one of an update version number and a build number of the client instance; and one of: a) the address system is configured to determine the endpoint address to be an address of a testing environment if the client instance is a beta version, or b) the address system is configured to determine the endpoint address to be a non-existent address if the client instance is determined to be behaving badly by an operator of a communication system which includes said address system. receive an address query from a device of the network, said address query being formed in dependence on a type of a client instance executed at the device, said address query being for requesting a network address of an endpoint (110 1) of the network; store relationships which map address queries to network addresses, wherein the stored relationships distinguish between address queries formed in dependence on different types of client instance; use the stored relationships to determine an endpoint address in the network based on the address query such that the determined endpoint address is dependent upon the type of the client instance executed at the device; and send an answer to the device in response to the address query, said answer including the determined endpoint address; the address system characterized in that the type of the client instance upon which the address query and endpoint address depend is given by one of an update version number and a build number of the client instance; and one of: a) the address system is configured to determine the endpoint address to be an address of a testing environment if the client instance is a beta version, or b) the address system is configured to determine the endpoint address to be a non-existent address if the client instance is determined to be behaving badly by an operator of a communication system which includes said address system. a) the address system is configured to determine the endpoint address to be an address of a testing environment if the client instance is a beta version, or b) the address system is configured to determine the endpoint address to be a non-existent address if the client instance is determined to be behaving badly by an operator of a communication system which includes said address system.