CompTIA Network+ Preparation · lesson 2 of 30 · 25 min
Why Ports Matter
Port numbers identify specific services on a host. When a packet arrives, the OS uses the destination port to deliver it to the correct application. The Network+ exam requires you to memorize well-known ports (0–1023) and know which transport protocol (TCP or UDP) each uses.
Must-Know Ports
TCP vs UDP — Which Protocols Use Which
Memory tip
UDP is used for services that prioritize speed over reliability: DNS, DHCP, TFTP, NTP, SNMP, Syslog. Everything else is typically TCP.
Flashcard Drill
Port 22 SSH — Secure Shell, encrypted remote access. TCP. Port 53 DNS — Domain Name System. UDP (TCP for zone transfers). Port 80 / 443 HTTP (80) unencrypted / HTTPS (443) encrypted web. Both TCP. Port 25 SMTP — sends email between servers. TCP. Port 110 / 143 POP3 (110) downloads email / IMAP (143) keeps on server. Both TCP. Port 67/68 DHCP — 67 server, 68 client. UDP. Port 161/162 SNMP — 161 queries, 162 traps. UDP. Port 3389 RDP — Remote Desktop Protocol. TCP. Port 123 NTP — Network Time Protocol. UDP. Port 514 Syslog — log forwarding. UDP.
Worked Scenario: Firewall Rule Analysis
Scenario
A security analyst reviews a firewall log and sees blocked traffic on port 23 from an internal host to an external IP. What protocol is being blocked, and why is this a security concern?
Answer: Port 23 is Telnet — an unencrypted remote access protocol. Blocking outbound Telnet is correct security practice because Telnet transmits all data including passwords in plaintext. An attacker with network access could capture credentials with a packet sniffer. The correct replacement is SSH (port 22), which encrypts the entire session.
Exam tip: When you see port 23 on the exam, the answer is almost always “use SSH instead” or “this is a security risk because it’s unencrypted.”
Knowledge Check — 12 Questions
Ports & Protocols (Network+)
Which port does SSH use?
- 21
- 22
- 23
- 25
Answer: 22
SSH uses TCP port 22 for encrypted remote access.
DNS primarily uses which transport protocol?
- TCP
- UDP
- Both equally
- Neither
Answer: UDP
DNS uses UDP port 53 for queries. TCP is used for zone transfers and large responses.
HTTPS uses port _____.
Answer: 443
HTTPS (HTTP over TLS) uses TCP port 443.
Which protocol uses ports 67 and 68?
- DNS
- DHCP
- SNMP
- NTP
Answer: DHCP
DHCP uses UDP port 67 (server) and 68 (client).
Which email protocol keeps messages on the server?
- SMTP (25)
- POP3 (110)
- IMAP (143)
- NNTP (119)
Answer: IMAP (143)
IMAP keeps email on the server and syncs across devices. POP3 downloads and typically deletes.
RDP uses which port?
- 22
- 23
- 3389
- 445
Answer: 3389
RDP (Remote Desktop Protocol) uses TCP port 3389.
SNMP traps are sent on which port?
- 161
- 162
- 514
- 123
Answer: 162
SNMP uses UDP 161 for queries and UDP 162 for traps (unsolicited alerts from devices).
Telnet (port 23) encrypts its traffic.
- True
- False
Answer: False
Telnet is unencrypted — all traffic including passwords is sent in plaintext. Use SSH instead.
Which protocol is used for Windows file sharing?
- FTP (21)
- SMB (445)
- TFTP (69)
- NFS (2049)
Answer: SMB (445)
SMB (Server Message Block) uses TCP port 445 for Windows file and printer sharing.
NTP (time synchronization) uses port _____.
Answer: 123
NTP uses UDP port 123 for time synchronization.
Which port does LDAP use for directory services queries?
- 389
- 636
- 443
- 445
Answer: 389
LDAP uses TCP port 389. LDAPS (LDAP over TLS) uses port 636.
A network admin needs to transfer a Cisco IOS image to a router with no authentication. Which protocol should they use?
- FTP (21)
- SFTP (22)
- TFTP (69)
- SCP (22)
Answer: TFTP (69)
TFTP (Trivial FTP) uses UDP port 69 and requires no authentication — commonly used for IOS image transfers in lab environments.
Previous lesson: OSI and TCP/IP models: layers, PDUs, encapsulation
Back to CompTIA Network+ Preparation
Next lesson: TCP three-way handshake, connection termination, UDP characteristics