Well so to speak, a kazillion ways to connect to a private network. Here are just a few of the common ones.

1. Dial-up via PPP, Slip, VPN, DUN etc....
2. VPN Via the Internet....
3. Password protected website....
4. Hidden directory....
5. Remote FTP....
6. Password protected hidden IP....
7. Telnet....
8. BBS (for us old timers)....
9. Dial-up BBS....

Many, many, many ways to connect. Thats why network security is so important. Just think of your house as a computer. You use the front door most of the time, the back door less. Then you have maybe a garage door, windows, even a pet door. So you have many ways in and depending on the persistance of the would be intruder they will find a way in. Maybe through a crawl space under the porch. A computer or network is the same way. Ports are the windows, doors and such. A firewall, good ruleset, permisions, and caution is your burglar alarm and locks. So you protect it like you protect your home.