Computerz R Us Education Material

IPv4 Addressing 32-bit

• Format: 4 octets (e.g., 192.168.1.10).
• Size: ~4.3 billion addresses (2³²).
• Written: dotted-decimal notation.
• Network / Host split: controlled by subnet mask (e.g., /24).

Classful Legacy Ranges

Modern networks use CIDR (Classless Inter-Domain Routing) instead of strict A/B/C classes.

Private IPv4 Ranges

IPv6 Overview 128-bit

• Format: 8 groups of 4 hex digits (e.g., 2001:0db8::1).
• Size: 3.4 × 10³⁸ addresses (basically unlimited).
• Shortening: leading zeros removed, one :: allowed.
• Prefix length: e.g., /64 for a typical subnet.
• Types: Global unicast, link-local, unique local, multicast.

APIPA (Automatic Private IP Addressing)

• Range: 169.254.0.0 – 169.254.255.255.
• Triggered when: Client cannot reach a DHCP server.
• Purpose: Allows limited local communication on same segment.
• No default gateway: No internet access with APIPA.
• Fix: Check DHCP, cabling, Wi-Fi, VLANs, IP conflicts.

If you see 169.254.x.x, think: “DHCP problem”.

Internet Access Types

• DSL: Uses phone lines, typically 5–100 Mbps down. Distance-sensitive.
• Cable: Coax via DOCSIS. 100–1200+ Mbps down, shared with neighbors.
• Fiber: Symmetric speeds, 100 Mbps – 10 Gbps+, low latency.
• Dial-Up: Legacy 56 kbps over analog phone lines. Very slow.
• Satellite: Wide coverage, high latency; weather sensitive.
• Cellular/Hotspot: 4G/5G using carriers (Verizon, T-Mobile, AT&T, etc.). Great backup/remote option.

Duplex Modes

• Half Duplex: Data flows one direction at a time (like a walkie-talkie).
• Full Duplex: Send and receive simultaneously (like a phone call).
• Collisions: Only an issue on half-duplex shared media (old hubs).
• Modern networks: Almost everything is full-duplex via switches.

Common Symptoms of Duplex Mismatch

• One end set to Auto, other hard-coded → half vs full mismatch.
• Slow file transfers despite good link speed.
• High CRC errors / late collisions on switch port.

Ethernet Speeds

Cat7/7a: shielded cable mainly for noisy or high-end environments (10G, some short higher-speed runs).
Cat8: heavily shielded, short-run (≈30m) data center cable for 25/40G links — not typically used in homes.

Which cable to pull?
• New home/small office: Cat6 or Cat6a (Cat6a if you want easy 10G later).
• Larger offices / new builds: Cat6a as the default standard.
• Specialty / noisy or shielded runs: Cat7/7a if the hardware actually supports it.
• Data center top-of-rack / super short high-speed copper: Cat8 or fiber.

Wi-Fi Standards & Bands

Bands & Channel Planning

• 2.4 GHz: Longer range, penetrates walls better, but only 3 non-overlapping channels (1, 6, 11) and lots of interference.
• 5 GHz: Shorter range, more channels, less interference; best for most laptops/phones in typical homes and offices.
• 6 GHz: Clean spectrum for Wi-Fi 6E/7; excellent in dense environments with modern devices.
• Channel width: 20/40/80/160 MHz:
  • 20 MHz – most stable; best in dense neighborhoods and apartment buildings.
  • 40/80 MHz – higher throughput, but more chance of overlap / interference.
  • 160 MHz – only when spectrum is very clean and hardware supports it.

*Speeds shown are theoretical PHY rates. Real usable throughput is much lower and shared among all clients on that AP.

Wi-Fi Security & Best Practices

Security Modes

• Open: No password. Only for guest hotspots with isolation and captive portal; avoid for anything sensitive.
• WEP: Obsolete and broken. Do not use.
• WPA (TKIP): Older, weak by today’s standards. Avoid if possible.
• WPA2-Personal (PSK, AES): Minimum acceptable for most home/small business setups.
• WPA3-Personal (SAE): Stronger protection against password guessing; preferred when supported.
• WPA2/WPA3-Enterprise: Uses 802.1X + RADIUS; per-user credentials/certs for organizations.

Extra Considerations

• WPS (Wi-Fi Protected Setup): Push-button / PIN convenience, but can be exploited. Many admins disable it.
• Guest Networks:
  • Use separate SSID/VLAN for guests.
  • Isolate guests from LAN and each other.
  • Optional captive portal for terms or vouchers.
• SSID & Password:
  • Use strong passphrases (12+ chars, random or passphrase style).
  • Avoid using company name + year as the password.
• Roaming & Coverage:
  • Multiple APs with overlapping but not identical coverage.
  • Same SSID/security across APs for smooth roaming.
  • Use proper channel planning (don’t stack all APs on channel 6).

Simple rule: WPA2-AES at minimum, WPA3 where possible, guests on their own VLAN, WPS off unless truly needed.

OSI Layers (Top to Bottom)

Common Mapping to Real Devices

• Firewalls: L3–L7 (routing, filtering, inspection).
• Routers: L3 (IP routing, VLAN routing).
• Switches: L2, some L3 (VLANs, inter-VLAN routing).
• Access Points (APs): L1–L2 (Wi-Fi radios & MAC handling).
• NICs: L1–L2 (physical link and MAC addressing).

When troubleshooting, ask “which layer is broken?” and move up/down systematically.

SIP & RTP

• SIP (Session Initiation Protocol): Sets up, changes, and tears down calls.
• RTP (Real-time Transport Protocol): Carries the actual voice media.
• SDP: Negotiates codecs, ports, IPs inside SIP messages.

VoIP Network Considerations

• QoS: Prioritize voice VLAN or DSCP markings.
• Jitter / Latency: Keep round-trip < 150 ms for good quality.
• Codecs: G.711 (higher bandwidth, better quality), G.729/Opus (compressed).
• NAT Traversal: STUN, TURN, SBCs often required.

Common Network Ports

Email Protocols & Ports

Modern setups: IMAPS (993) for reading, SMTP submission (587) for sending.

Backup Types

• Full Backup: Copies everything selected each time.
• Incremental: Copies only changes since the last any backup (full or incremental).
• Differential: Copies changes since the last full backup.

Incremental vs Differential

Local vs Cloud Backups

Local Backups

• External drives, NAS, on-prem backup appliances.
• Fast restores for large amounts of data.
• But vulnerable to theft, fire, ransomware if always online.

Cloud Backups

• Data stored off-site in a provider’s data center.
• Good for disasters and off-site resilience.
• Restores limited by internet bandwidth and size.

Auth Factors & Options

• Something you know: Passwords, PINs, security questions.
• Something you have: Phone, hardware token, smart card, FIDO key.
• Something you are: Fingerprint, FaceID, biometrics.

Common MFA / 2FA Methods

• SMS Codes: One-time code sent by text. Better than nothing but vulnerable to SIM-swap and phishing.
• TOTP Apps: Time-based codes (Google Authenticator, Microsoft Authenticator, Authy). More secure than SMS.
• Push Approvals: “Approve / Deny” prompts on an app. Easy for users but watch for push fatigue.
• Hardware Tokens: FIDO2/U2F keys (YubiKey, etc.) or smart cards. Very strong, good for admins and high-risk accounts.

Passkeys & Best Practices

Passkeys (FIDO2/WebAuthn)

• Passwordless login using device-based cryptographic keys.
• Protected with biometrics or device PIN (phone, laptop, security key).
• Resistant to phishing — no code to type into a fake site.

Deployment Tips

• Enable MFA at least for:
  • Admin accounts (AD, cloud, firewalls, routers).
  • Email accounts (O365, Google Workspace, etc.).
  • Remote access (VPN, RDP gateways, remote tools).
• Prefer app-based codes or passkeys over SMS when possible.
• Train users not to approve random push requests (MFA fatigue attacks).
• Store backup recovery codes securely (password manager, locked safe).

Quick order of strength (weakest → strongest): Password only < SMS code < Authenticator app < Hardware key / Passkey.

Single-Mode vs Multi-Mode

Single-Mode (SMF)

• Very small core (~9µm), lasers as light source.
• Long distances: 10 km to 80+ km (WAN, long building runs).
• Jacket color often yellow.

Multi-Mode (MMF)

• Larger core (50/62.5µm), LEDs/VCSELs.
• Shorter distances: typically up to 300–400 m for 10G.
• Common data center and campus runs.

Fiber Types & Typical Use

Connectors: LC is most common now; SC/ST seen in older gear.

Northbridge (Memory / High-Speed Hub)

• Sits closest to the CPU on the motherboard.
• Handles high-speed communication between:
  • CPU <-> RAM (memory controller).
  • CPU <-> high-speed graphics (AGP / PCI Express x16).
  • CPU <-> Southbridge (downstream link).
• Directly affects:
  • Maximum supported RAM type and speed (DDR versions, timings).
  • Number and speed of PCIe lanes for GPUs and high-speed cards.
  • Front Side Bus (FSB) speed on older Intel platforms.
• Runs hotter and may need its own heatsink because of high-speed traffic.

Think of the Northbridge as the “fast lane” traffic controller for CPU, RAM, and GPU in classic PC designs.

Southbridge (I/O & Legacy Hub)

• Connects slower peripherals and legacy devices to the rest of the system.
• Traditionally handled:
  • PCI and PCIe x1 slots (low-speed expansion cards).
  • SATA / IDE controllers for hard drives and optical drives.
  • USB ports, PS/2 keyboard/mouse ports.
  • Onboard audio, network (LAN), and miscellaneous controllers.
  • CMOS/BIOS, real-time clock, some power management.
• Connected upstream to the Northbridge via a chipset link (e.g., DMI, proprietary bus).
• Lower bandwidth than Northbridge but supports many more devices.

The Southbridge is the “I/O hub” that ties together storage, USB, and onboard devices.

Key Directories

• /bin – Essential user binaries (e.g., ls, cp, mv).
• /sbin – System binaries (e.g., fsck, iptables).
• /etc – System-wide configuration files.
• /home – User home directories.
• /opt – Optional or third-party software.
• /tmp – Temporary files, often cleared at reboot.
• /usr – User programs, libraries, documentation.
• /var – Variable data (logs, mail, spool, DBs).
• /dev – Device files (disks, terminals, etc.).
• /proc – Virtual filesystem exposing kernel & process info.

File Types

• Regular file (-): Text, binaries, media.
• Directory (d): Folder containing other files.
• Character device (c): Serial devices, keyboards.
• Block device (b): Disks, partitions.
• Symbolic link (l): Pointer to another path.
• Socket (s): For IPC and networking.
• FIFO/Named pipe (p): One-way inter-process communication.

Level 1 (L1) – Front Line

• First contact via phone, email, or ticket.
• Handle password resets, basic troubleshooting, how-to questions.
• Use scripts and knowledge base articles.
• Gather logs and details for more complex issues.
• Escalate when beyond scope or time limit.

Level 2 (L2) – Technical Specialists

• Deeper OS, network, and application troubleshooting.
• Work with servers, AD, group policies, VPN, printing, etc.
• Analyze logs and patterns, perform remote sessions.
• Implement non-standard fixes and changes.

Level 3 (L3) – Experts / Engineering

• Senior admins, network engineers, developers.
• Handle complex outages, design issues, and bugs.
• Work with vendors on escalated support cases.
• Design improvements, standards, and automations.

Lens Types & Usage

• 2.8 mm (Wide): Very wide FOV (~100°). Great for doorways, porches, small yards.
• 4 mm (Medium): Balanced FOV (~80°). Good for driveways, small parking areas.
• 6–12 mm (Tele): Narrow FOV (~30–50°). Good for gates, far corners, license plates.
• Varifocal (e.g., 2.8–12 mm): Adjustable zoom to tune the scene after mounting.

Rule of thumb: wide at doors & entries, telephoto for long driveways & plates.

Field of View Diagram