There are many protocols and port numbers that a Network Engineer could be expected to know. Here are the most important 18 protocols for you, with a short description of each.
Before we begin, it is important to understand that there are two main types of protocols. They are the UDP "connectionless" (broadcast) protocols and the TCP connection-oriented protocols.
UDP stands for User Datagram Protocol. For each of these, packet-loss is not a big deal. For example, VOIP/Skype calls fall under UDP. In this case if you lose a few voice or image packets the call can still continue. If we prioritized every packet highly, we'd get dropped calls all the time.
TCP stands for Transport Control Protocol. These are connection-oriented. A 3-way handshake is needed to initiate a connection and in this case each packet is vital; any lost packets must be re-sent. Consider downloading an image via HTTP (to be rendered onto a webpage). In this case it's okay to receive packets out of order, but you'll never get to see the complete image if you don't ensure that all packets have been received.
Finally, before we jump into the list, know that some protocols are counted as both TCP and UDP. These are listed separately at the end.
Essential UDP Protocols
SNMP (Simple Network Management Protocol), Ports 161, 162: manage and monitor systems in your network
NTP (Network Time Protocol), Port 123: time synchronization
SIP (Session Initiation Protocol), Ports 5060, 5061-TLS: video and voice
RTSP (Real-Time Streaming Protocol), Port 554: streaming media
DHCP (Dynamic Host Configuration Protocol), Ports 67, 68: provide IP addresses to PCs on network, as well as Subnet Mask, Default Gateway, and DNS Servers
TFTP (Trivial File Transfer Protocol), Port 69: no authentication required
Essential TCP Protocols
FTP (File Transfer Protocol), Ports 20-send/receive, 21-establish: password-based authentication
SFTP (Secure File Transfer Protocol), Port 22: encrypted file transfer
SSH (Secure Shell), Port 22: encrypted text-based remote management
Telnet, Port 23: text-based remote management, insecure (prefer SSH)
SMTP (Simple Mail Transfer Protocol), Ports 25, 465-TLS: inter-server communication for email services
IMAP4 (Internet Message Access Protocol), Ports 143, 993-TLS: client downloads copy of message
POP3 (Post Office Protocol), Ports 110, 995-TLS: client downloads original message
HTTP (Hypertext Transfer Protocol), Port 80: unencrypted web traffic
HTTPS (Hypertext Transfer Protocol Secure), Port 443-TLS: encrypted web traffic
Essential Hybrid TCP/UDP Protocols
LDAP (Lightweight Directory Access Protocol), Port 389: share user IDs, work with Active Directory
RDP (Remote Desktop Protocol), Port 3389: Windows GUI-based remote management
DNS (Domain Name System), Port 53: name-based site lookup
Network Engineer Academy Video: 'The 18 Protocols You Should Know For Your IT Career!'