If a helper application is associated with the given media type, it is started by the Navigator. A helper application can be any software program that runs on your system. Many helpers have nothing to do with networking. Consider using Microsoft Paint as a bitmap viewer. Navigator encounters a bitmap file, downloads it, and opens Paint with the given filename.
Paint operates on the local file normally.
A good example of a sockets-based helper application is Progressive Networks' RealAudio Player, which is shown in Figure 4.1. The RealAudio Player plays real-time sound over the Internet. The RealAudio Player from Progressive Networks. These helpers can perform all network duties, such as loading URLs and writing to Web servers, strictly through the Navigator. This network is, of course, the Internet.