Network Services and Protocols

Week of 2026-09-29 · Download .docx

Objectives

Identify common network ports and their associated protocols
Explain DNS, DHCP, and VPN functions
Use Windows network troubleshooting commands

Key terms

SSH: Secure Shell — TCP port 22; encrypted remote terminal access replacing Telnet.
DNS: Domain Name System — port 53; translates hostnames to IP addresses.
RDP: Remote Desktop Protocol — TCP port 3389; graphical Windows remote access.
HTTPS: HTTP Secure — port 443; web traffic encrypted with TLS.
SMTP: Simple Mail Transfer Protocol — port 25; outbound email delivery between mail servers.
IMAP: Internet Message Access Protocol — port 143; email retrieval keeping messages on server.
POP3: Post Office Protocol 3 — port 110; downloads email and typically removes from server.
ICMP: Internet Control Message Protocol — used by ping; Layer 3 diagnostic protocol.
TLS: Transport Layer Security — protocol encrypting HTTPS and other application traffic.
VPN: Virtual Private Network — encrypted tunnel over the public internet for secure remote access.
ipconfig: Windows command displaying IP address, mask, gateway, and MAC for all adapters.
nslookup: Command-line tool querying DNS servers to verify hostname-to-IP resolution.

The concept

PORT NUMBERS AS ADDRESSES

Every network service listens on a specific TCP or UDP port number. The port tells the OS which application should receive incoming data. Memorizing the common ports is a core A+ competency because they appear in firewall rules, troubleshooting, and security configurations: HTTP=80, HTTPS=443, SSH=22, Telnet=23, FTP=20/21, SMTP=25, DNS=53, DHCP=67/68, RDP=3389, POP3=110, IMAP=143.

HOW DNS WORKS

When you type a website name, your computer asks its configured DNS server to look up the IP address. The DNS server returns the answer, and your computer caches it for a period defined by the TTL (Time to Live). If the local cache contains a stale entry, running 'ipconfig /flushdns' clears it and forces a fresh lookup. The tool 'nslookup' lets you query a DNS server directly to verify a record.

EMAIL PROTOCOLS

SMTP handles outgoing mail — it relays messages between mail servers and from client to server. POP3 downloads messages to a local client and typically removes them from the server, meaning you can only access mail on one device. IMAP keeps messages on the server and synchronizes state (read/unread/folders) across all devices — the preferred protocol for modern multi-device access.

VPN FUNDAMENTALS

A VPN creates an encrypted tunnel between the client and a VPN server, typically at the corporate office. All traffic from the client travels through this tunnel before reaching the internet, making it appear to originate from the VPN server's IP. This protects data in transit on untrusted networks (public Wi-Fi) and enables secure access to internal corporate resources from any location.

WINDOWS NETWORK COMMANDS

ipconfig displays current IP settings and supports /release, /renew, /flushdns. ping tests reachability using ICMP. nslookup queries DNS. tracert maps hop-by-hop routing. netstat shows active connections and listening ports.

Worked examples

Example 1: A user cannot access email on their phone after switching carriers. They report new messages are missing. Diagnosis: the phone is configured for POP3, which downloads and deletes messages from the server. Switching to IMAP ensures messages stay on the server and sync across all devices.

Example 2: A technician opens a firewall rule for SSH access to a server. They must know to open TCP port 22 inbound. They also need to allow DNS outbound (UDP/TCP port 53) for the server to resolve hostnames.

Common mistakes

Confusing POP3 (downloads and removes) with IMAP (syncs to server) — wrong choice causes missing emails.
Remembering HTTP=443 and HTTPS=80 — these are reversed; HTTP=80, HTTPS=443.
Confusing SSH (port 22, terminal access) with RDP (port 3389, graphical desktop).
Forgetting that DNS uses both UDP and TCP on port 53 — UDP for standard queries, TCP for large responses.
Using ifconfig instead of ipconfig on Windows — ifconfig is Linux/Unix; ipconfig is Windows.

Self-check

Try each question before reading the answer. Answers at the bottom of this page.

1. SSH uses which default port?

2. Which email protocol leaves messages on the server?

IMAP (port 143)
POP3 (port 110)
SMTP (port 25)
FTP (port 21)

3. HTTPS encrypts web traffic using:

TLS
SSH
IPsec
SFTP

4. 'ipconfig /flushdns' clears:

Local DNS resolver cache
DHCP lease
ARP cache
Routing table

5. RDP uses which port?

3389
22
443
8080

Self-check answers

1. A — SSH (Secure Shell) uses TCP port 22 for encrypted remote terminal access.
2. A — IMAP keeps messages on the server and synchronizes across all devices.
3. A — HTTPS wraps HTTP inside a TLS session for encryption and server authentication.
4. A — flushdns clears the local DNS resolver cache, forcing fresh DNS lookups.
5. A — Remote Desktop Protocol uses TCP port 3389 for graphical Windows remote access.

Canvas is the official record. This companion enhances the PGCC curriculum; it does not replace it. Last name and class year only. Students with a 504 plan or IEP: your accommodations apply.

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