Note 4.3.1.
For an interesting example of how you can change but still follow a protocol, check out SYN cookies.
1
en.wikipedia.org/wiki/SYN_cookies
en.wikipedia.org/wiki/SYN_cookies
Port Number | L4 Protocol | Usage |
20 | TCP | File Transfer Protocol (FTP) Data Transfer |
21 | TCP | FTP Command Control |
22 | TCP | Secure Shell (SSH) |
23 | TCP | Telnet Remote Login Service |
25 | TCP | Simple Mail Transfer Protocol (SMTP) E-Mail |
53 | TCP, UDP | Domain Name System (DNS) |
67, 68 | UDP | Dynamic Host Configuration Protocol (DHCP) |
69 | UDP | Trivial File Transfer Protocol (TFTP) |
80 | TCP | Hypertext Transfer Protocol (HTTP) |
110 | TCP | Post Office Protocol (POP3) E-Mail |
119 | TCP, UDP | Network News Transfer Protocol (NNTP) |
123 | UDP | Network Time Protocol (NTP) |
137–139 | TCP, UDP | NetBIOS |
143 | TCP | Internet Message Access Protocol (IMAP) E-Mail |
161, 162 | TCP, UDP | Simple Network Management Protocol (SNMP) |
194 | TCP, UDP | Internet Relay Chat (IRC) |
389 | TCP, UDP | Lightweight Directory Access Protocol (LDAP) |
443 | TCP | HTTP Secure (HTTPS) — HTTP over TLS/SSL |
3389 | TCP, UDP | Microsoft Terminal Server (RDP) |