CompTIA A+ Cram Sheet — Quick Reference

OSI Model (7 Layers)

#	Layer	Protocols/Devices
7	Application	HTTP, FTP, DNS, SMTP, SNMP
6	Presentation	SSL/TLS, JPEG, encryption
5	Session	NetBIOS, RPC, SQL sessions
4	Transport	TCP (reliable), UDP (fast)
3	Network	IP, ICMP, routers
2	Data Link	MAC, Ethernet, switches, ARP
1	Physical	Cables, hubs, signals, Wi-Fi

Mnemonic (top-down): All People Seem To Need Data Processing. (bottom-up): Please Do Not Throw Sausage Pizza Away.

Common Network Ports

Port	Protocol	Use
20/21	FTP	File Transfer (20=data, 21=control)
22	SSH	Secure remote access
23	Telnet	Unsecure remote access
25	SMTP	Send email
53	DNS	Name resolution (TCP+UDP)
67/68	DHCP	IP address assignment
80	HTTP	Web (unencrypted)
110	POP3	Receive email (downloads)
143	IMAP	Receive email (syncs)
443	HTTPS	Web (encrypted/TLS)
3389	RDP	Remote Desktop Protocol

IP Address Ranges

Class	Range	Default Subnet
Class A	1.0.0.0 – 126.255.255.255	255.0.0.0 (/8)
Class B	128.0.0.0 – 191.255.255.255	255.255.0.0 (/16)
Class C	192.0.0.0 – 223.255.255.255	255.255.255.0 (/24)

Private Ranges (not routable)
10.0.0.0 – 10.255.255.255 (Class A)
172.16.0.0 – 172.31.255.255 (Class B)
192.168.0.0 – 192.168.255.255 (Class C)
169.254.x.x = APIPA (DHCP failed)
127.0.0.1 = Loopback (localhost)

Windows Command Line Tools

Command	Purpose
`ipconfig`	Show IP config; /all for details; /release & /renew for DHCP
`ping`	Test connectivity to host
`tracert`	Trace route to destination (shows hops)
`netstat`	Show active connections and listening ports
`nslookup`	DNS lookup — query name or IP
`sfc /scannow`	System File Checker — repairs corrupted Windows files
`chkdsk`	Check and repair disk errors
`diskpart`	Disk partitioning utility
`gpupdate`	Force Group Policy update

RAID Levels

RAID	Method	Min Drives	Fault Tolerance
RAID 0	Striping only	2	None — one failure = total loss
RAID 1	Mirroring	2	1 drive failure
RAID 5	Striping + Parity	3	1 drive failure
RAID 6	Striping + Dual Parity	4	2 drive failures
RAID 10	Stripe of Mirrors	4	1 per mirror pair

RAID is NOT a backup! RAID 0 = speed (no redundancy). RAID 1 = redundancy (50% capacity). RAID 5 = balance. RAID 10 = best overall.

6-Step Troubleshooting Methodology

Identify the problem — gather info, symptoms, recent changes
Establish a theory — probable cause (question obvious first)
Test the theory — confirm cause; if wrong, establish new theory
Establish plan of action — resolve the issue, consider implications
Implement solution — fix the problem, escalate if needed
Verify and document — confirm full functionality, document findings

Laptop Display Types

Type	Pros	Cons
TN	Fast response, cheap	Poor viewing angles, colors
IPS	Great colors, wide angles	More expensive, slight ghosting
OLED	Perfect blacks, vibrant	Burn-in risk, expensive

Security Quick Reference

Threat	Description	Prevention
Phishing	Fraudulent emails to steal credentials	User training, email filtering, MFA
Ransomware	Encrypts files, demands payment	Offline backups, patching, AV, email filtering
Social Engineering	Manipulating humans to bypass security	Security awareness training
Zero-Day	Exploit targeting unpatched vulnerability	Behavior-based AV, network segmentation, fast patching
Tailgating	Physical unauthorized entry behind authorized person	Security policies, access badges, mantraps

Encryption: BitLocker = full disk (needs TPM). EFS = individual files/folders. Always save BitLocker recovery key OFF the encrypted drive.

Principle of Least Privilege: Grant minimum permissions needed. Run as standard user daily. Admin only when required. Reduces attack surface significantly.