What are port numbers?
As we were discussing the OSI model and the difference between TCP and UDP we already learned some of the bits how protocols work on the transport layer. Together with the transport protocol used (these are not limited to TCP and UDP only but these are the ones we will talk about today), each of them uses a port number. These ports get assigned by IANA and although you will be always able to look them up on the internet the is a (‘the’) list of well known port numbers.
Do I have to learn that list by heart?
No, you don’t but it can make your troubleshooting life a lot easier if you do. You can always compile a list of your own, but here is my list:
| Protocol | Transport Protocol/Port |
| http | tcp/80 |
| https | tcp/443 |
| telnet | tcp/23 |
| ssh | tcp/22 |
| ftp | tcp/21 |
| smtp (unencrypted) | tcp/25 |
| smtp (encrypted) | tcp/465 |
| ntp | udp/123 |
| snmp read | udp/161 |
| snmp trap | udp/162 |
| ldap | tcp/389 |
| ldaps (secure) | tcp/363 |
| pop | tcp/110 |
| rdp | tcp/3389 |
| imap (unencrypted) | tcp/143 |
| imap (encrypted) | tcp/993 |
| dns (queries) | udp/53 |
| dns (zone transfers) | tcp/53 |
| tftp | udp/69 |
| syslog | tcp/udp/514 |
For a complete list, look here
