From Programmed Integration:
Acts as both client and server program. IPUP will sit quite happily on your system tray and even email you when an IP notification is received, ideal if you are away from the office and need to easily get your clients IP address.
By purchasing a single license you are entitled to run one server and one client. With a site license you are free to distribute it within your company and to your clients. With a world license you are unrestricted.