Any program with an IP stack can/does open up port/s and broadcast, ports are 'the pipe'. You have to open a port before you can send anything over an IP based 'network' port. That's what UPnP is doing, it's up to the/a device to be listening to finish the 'whatever' that UPnP is requesting.
Here is a lot better explaination than mine:
he UPnP Device Architecture (UDA) is more than just a simple extension of the plug and play peripheral model. It is designed to support zero-configuration, "invisible" networking, and automatic discovery for a breadth of device categories from a wide range of vendors. This means a device can dynamically join a network, obtain an IP address, convey its capabilities, and learn about the presence and capabilities of other devices. Finally, a device can leave a network smoothly and automatically without leaving any unwanted state behind