The present invention allows a client on a private network to determine changes in a public address, which is provided by an address translation device and used for communications on a public network. The client will receive messages over the private network from the public network via the address translation device. In the message, a copy of the public address is placed in a portion of the message. When the address translation device modifies the message such that the client's private address is used for delivery of the message to the client, the copy of the public address remains in the message. Upon receipt, the client will analyze the message to identify the public address and compare the public address with a previously known public address. If the public address provided in the message is different from the stored public address, the client can recognize that it has changed.