not rules, but an agreed set of standards
| Protocol | Port | Application |
|---|---|---|
| TCP | 80 | HTTP |
| TCP | 443 | HTTPS |
| UDP | 53 | DNS |
| TCP | 21 | FTP |
| UDP | 67/68 | DHCP |
| TCP/UDP | 25 | SMTP |
| UDP | 161 | SNMP |
| TCP | 22 | SSH |
| TCP | 110 | POP3 |
| TCP | 143 | IMAP |
| TCP/UDP | 67/68 | DHCP |
| TCP | 389 | LDAP |
| TCP | 443 | LDAPs |
| TCP | 3389 | RDP |
| TCP/UDP | 123 | NTP |