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Resource Cheatsheet

A+ Core 1 2.1: Common Ports

Professor Messer A+ 220-1201 objective 2.1 — FTP, SSH, Telnet, SMTP, DNS, DHCP, HTTP/HTTPS, POP3, IMAP, SMB/CIFS, NetBIOS, LDAP and RDP port numbers.

Updated
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4 min read
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A+ Core 1
Detail
Common Ports
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In one line: know the port, the protocol (TCP/UDP), and why each service matters — for troubleshooting and for firewall rules.

Companion to: Professor Messer, CompTIA A+ 220-1201 Core 1, objective 2.1 (common ports). The full version is the Common Ports class notes; port fundamentals are in the Section 2 primer sheet.

The port list

Port Proto Service One-line role
20 TCP FTP (data) Active data transfer
21 TCP FTP (control) Administration / control connection
22 TCP SSH Encrypted remote command line
23 TCP Telnet Unencrypted remote command line — avoid
25 TCP SMTP Sends email, server-to-server and device-to-server
53 UDP DNS Resolves FQDN → IP address
67 UDP DHCP (server) Hands out IP configuration
68 UDP DHCP (client) Client side of DHCP
80 TCP HTTP Web, unencrypted
110 TCP POP3 Downloads mail to one device
137 UDP NetBIOS Name Service Legacy Windows name resolution
139 TCP NetBIOS Session Service Legacy Windows file-transfer sessions
143 TCP IMAP4 Syncs mail inbox across devices
389 TCP LDAP Queries a directory service (e.g. Active Directory)
443 TCP HTTPS Web, encrypted
445 TCP SMB (direct) Modern Windows file sharing, no NetBIOS
3389 TCP RDP Remote desktop view/control

Beyond this lesson: LDAPS = 636; secure POP3/IMAP = 995/993; SMTP submission = 587.

Pairs worth remembering

Unencrypted Encrypted
Telnet (23) SSH (22)
HTTP (80) HTTPS (443)
LDAP (389) LDAPS (636 — beyond this lesson)
Sends mail Receives mail
SMTP (25) POP3 (110, one device) / IMAP (143, synced across devices)

DHCP terms

Pool (available addresses) → lease (temporary assignment, renew or release) → reservation (always the same IP for one device, e.g. a printer).

Windows file sharing

SMB / CIFS = Windows’s sharing protocol. Legacy NetBIOS: UDP 137 (names), TCP 139 (sessions). Modern direct / NetBIOS-less: TCP 445.

LDAP structure

Organisation → organisational units (e.g. Production, Support, Engineering) → common names (e.g. Sam, Daniel) → named resources (e.g. a database). Classic example: Microsoft Active Directory.

🔐 Security notes

  • Always prefer the encrypted option: SSH > Telnet; HTTPS > HTTP; LDAPS > LDAP.
  • Never expose RDP (3389) to the internet — one of the most attacked ports. Use a VPN or gateway, and MFA.
  • Telnet sends credentials in the clear — isolate any legacy device that still needs it.
  • DHCP and DNS are trust points: rogue DHCP servers and hijacked DNS settings redirect users silently. Managed switches use DHCP snooping.
  • Don’t expose SMB (445) to the internet — a common worm entry point historically.
  • Check what’s actually listening: netstat -an or Get-NetTCPConnection -State Listen.

Practice drills

1. A user can browse http:// sites but not https:// sites. Which port is blocked?

443 (HTTPS is blocked; HTTP/80 still works).

2. An admin needs an encrypted command-line session to a Linux server. Which protocol and port?

SSH, TCP 22.

3. An email client downloads mail and removes it from the server. Which protocol?

POP3 (TCP 110).

4. An email client keeps mail synced across a phone, laptop and webmail. Which protocol?

IMAP4 (TCP 143).

5. PCs on a network get no IP address automatically. Which ports would you check?

UDP 67 (DHCP server) and 68 (DHCP client).

6. A PC can reach 8.8.8.8 but not google.com. Which port/service?

UDP 53 — DNS.

7. Mapping \\fileserver\share fails only when a firewall rule blocks one port. Which port?

TCP 445 (direct SMB).

8. What port does Active Directory get queried on via LDAP?

TCP 389.

Key takeaways

  • FTP: 20 (data) / 21 (control). SSH 22 beats Telnet 23 (unencrypted).
  • SMTP 25 sends; POP3 110 downloads; IMAP4 143 syncs.
  • DNS UDP 53 resolves names; DHCP UDP 67/68 auto-configures IPs (pool → lease → reservation).
  • HTTP 80 vs HTTPS 443 — encryption is the difference.
  • SMB/CIFS for Windows sharing; legacy NetBIOS 137/139; modern direct 445.
  • LDAP 389 queries a directory service like Active Directory.
  • RDP 3389 — never expose to the internet directly.

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