Ports are like doors for a special service on a server or PC. They are identified by numbers from 0 to 65535. The well known standard ports are from 0 to 1024.
If a censor blocks a port every traffic on this port is dropped so its useless for you. Most censors block the ports 80, 1080, 3128, and 8080 because these are the common proxy ports. Because all of the proxies on these common ports are useless for you you have to find proxies that are listening on an uncommon port. These are very difficult to find.
You can easily test which ports are blocked on your connection using Telnet. Just open the DOS-Prompt, type "telnet login.icq.com 80" or "telnet login.oscar.aol.com 80" and hit enter. The number is the port you want to test. If you get some wired symbols in return everything is ok. If it says "timeout" or something similar that port is blocked by your ISP.
The most important ports:
- 20 and 21 - FTP file transfer
- 22 - SSH secure shell remote access
- 23 - Telnet unsecure remote access and also wingates, a special kind of proxies
- 25 - SMTP send email
- 53 - DNS resolves an URL to an IP
- 80 - HTTP normal web browsing and also a proxy
- 110 - POP3 receive email
- 143 - IMAP send/receive email
- 443 - SSL secure HTTPS connections
- 993 - secure IMAP
- 995 - secure POP3
- 1080 - Socks proxy
- 3128 - Squid proxy
- 8000 - Junkbuster proxy
- 8080 - Standard proxy
In my university for example, only the ports 22 (SSH), 110 (POP3), 143 (IMAP), 993 (secure IMAP), 995 (secure POP3) and 5190 (ICQ) are open for external connections. Every other port is either blocked or only available through a proxy. So that ports may be a good idea to run a proxy service.