Use the Internet Ports Database Web site to track down what it might be. The Web interface is useful, but the Domain Name System interface is extremely clever for a quick lookup. A 75-cent discrepancy in accounting records starts the author on an investigation that uncovers hackers in his system who appear to be participating in international espionage. An astronomer who is initially unfamiliar with computer security, his experiences guide both experienced professionals and newcomers through some of the challenges of the field, without getting bogged down in technical details. Any computer system today has users, and people never do what the designers expect.
Humans are inquisitive and inventive, yet lazy and ignorant.
If they're not downright malicious, nearly all are incompetent outside their particular area of skill. They also like to fiddle and play, leading to computer systems that seem bound to fail. Like everyone else, I enjoy exploring the possibilities of a system. I've sometimes even caused what could genuinely be called security incidents. But in the process, I've learned some valuable lessons.