A method of authorising data traffic originating from a client device to use a communications system, the method comprising the steps of: at an access point providing a wireless local area network: receiving a request for authorisation from the client device generating an authorisation request identifying the client device for transmission to an authentication server transmitting to an authentication server a message including meta-data identifying the client device and including in a source address field of the message an IP address allocated to the device wherein the message is subjected to network address translation NAT characterised in that at the authentication server the method comprises the steps of: receiving the NAT-translated message transmitted by the access point, the communication including a network address translated NAT IP source address extracting, from a source address field of the received communication, the IP source address which has undergone network address translation; recording in a data structure an association between the extracted translated IP source address and a client device from the information provided in said meta-data message; and authenticating the device as authorised to access said communications system at an access point providing a wireless local area network: receiving a request for authorisation from the client device generating an authorisation request identifying the client device for transmission to an authentication server transmitting to an authentication server a message including meta-data identifying the client device and including in a source address field of the message an IP address allocated to the device wherein the message is subjected to network address translation NAT characterised in that at the authentication server the method comprises the steps of: receiving the NAT-translated message transmitted by the access point, the communication including a network address translated NAT IP source address extracting, from a source address field of the received communication, the IP source address which has undergone network address translation; recording in a data structure an association between the extracted translated IP source address and a client device from the information provided in said meta-data message; and authenticating the device as authorised to access said communications system receiving a request for authorisation from the client device generating an authorisation request identifying the client device for transmission to an authentication server transmitting to an authentication server a message including meta-data identifying the client device and including in a source address field of the message an IP address allocated to the device wherein the message is subjected to network address translation NAT characterised in that at the authentication server the method comprises the steps of: receiving the NAT-translated message transmitted by the access point, the communication including a network address translated NAT IP source address extracting, from a source address field of the received communication, the IP source address which has undergone network address translation; recording in a data structure an association between the extracted translated IP source address and a client device from the information provided in said meta-data message; and authenticating the device as authorised to access said communications system characterised in that at the authentication server the method comprises the steps of: receiving the NAT-translated message transmitted by the access point, the communication including a network address translated NAT IP source address extracting, from a source address field of the received communication, the IP source address which has undergone network address translation; recording in a data structure an association between the extracted translated IP source address and a client device from the information provided in said meta-data message; and authenticating the device as authorised to access said communications system receiving the NAT-translated message transmitted by the access point, the communication including a network address translated NAT IP source address extracting, from a source address field of the received communication, the IP source address which has undergone network address translation; recording in a data structure an association between the extracted translated IP source address and a client device from the information provided in said meta-data message; and authenticating the device as authorised to access said communications system