Networking Fundamentals

Week of 2026-09-15 · Download .docx

Objectives

Map the 7 OSI layers to real protocols and explain encapsulation and decapsulation as data travels down and up the stack.
Calculate IPv4 addresses, subnet masks, and CIDR prefixes and identify RFC 1918 private ranges.
Match common protocols to their port numbers and describe TCP's three-way handshake vs. UDP's connectionless model.
Select the correct network device (hub, switch, router, WAP, firewall) for a given scenario and explain its OSI layer.

Key terms

OSI Model: Open Systems Interconnection — 7-layer reference model (Physical, Data Link, Network, Transport, Session, Presentation, Application) for standardizing network communications.
Encapsulation: The process of wrapping data with protocol headers (and trailers) at each OSI layer as it travels from Application (Layer 7) down to Physical (Layer 1).
MAC Address: 48-bit hardware identifier burned into a NIC. Used at Layer 2 (Data Link) for frame delivery within a local network segment.
IP Address: 32-bit (IPv4) or 128-bit (IPv6) logical address used at Layer 3 (Network) to identify devices across routed networks.
Subnet Mask: A 32-bit value that divides an IP address into its network portion (masked bits) and host portion (unmasked bits).
DHCP: Dynamic Host Configuration Protocol — automatically assigns IP address, subnet mask, default gateway, and DNS server to network clients.
DNS: Domain Name System — distributed database translating domain names (example.com) to IP addresses using UDP/TCP port 53.
TCP: Transmission Control Protocol — connection-oriented, reliable Layer 4 protocol using a three-way handshake (SYN→SYN-ACK→ACK) for ordered delivery.
UDP: User Datagram Protocol — connectionless, unreliable Layer 4 protocol with lower overhead, used for streaming, DNS, and latency-sensitive applications.
Default Gateway: The router's IP address on the local subnet. Devices send all packets destined outside the local subnet to the default gateway.
Switch: Layer 2 device that forwards Ethernet frames using a MAC address table, creating individual collision domains per port.
Router: Layer 3 device that forwards packets between networks using IP addresses and routing tables.
APIPA: Automatic Private IP Addressing — the OS self-assigns a 169.254.x.x address when DHCP fails, indicating a DHCP server problem.
ARP: Address Resolution Protocol — maps a known IPv4 address to a MAC address within the local network segment.

The concept

Networking is the plumbing of the IT world. Understanding how data moves from one device to another — and why — is foundational to every IT role from help desk to security analyst.

**The OSI Model**

The 7-layer OSI (Open Systems Interconnection) model provides a common vocabulary for networking. Each layer has a specific responsibility and communicates with the layer above and below it using well-defined interfaces. Mnemonic bottom-to-top: "Please Do Not Throw Sausage Pizza Away" (Physical, Data Link, Network, Transport, Session, Presentation, Application).

Layer 1 (Physical) transmits raw bits as electrical signals, radio waves, or light pulses. Layer 2 (Data Link) delivers frames on a local network segment using MAC addresses — Ethernet switches operate here. Layer 3 (Network) routes packets between networks using IP addresses — routers operate here. Layer 4 (Transport) provides end-to-end delivery between processes using TCP or UDP. Layers 5–7 manage sessions, data formatting, and application-specific protocols like HTTP, DNS, and SMTP.

**IP Addressing**

IPv4 addresses are 32 bits written as four decimal octets (192.168.1.10). The subnet mask defines where the network portion ends and the host portion begins. The three RFC 1918 private ranges — 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 — are not routed on the public internet and require NAT for external communication.

DHCP automates the four essential network configuration parameters: IP address, subnet mask, default gateway, and DNS server. Without these, a device cannot communicate. If DHCP fails, Windows assigns an APIPA address in the 169.254.0.0/16 range — a visible symptom of DHCP server unavailability.

**TCP vs. UDP**

TCP establishes a reliable connection using the three-way handshake: SYN (client initiates) → SYN-ACK (server acknowledges) → ACK (client confirms). After this, TCP provides sequenced, acknowledged delivery with automatic retransmission of lost segments. This reliability adds overhead, making TCP appropriate for file transfers, web browsing, and email.

UDP skips the handshake and does not acknowledge or retransmit. This reduces overhead significantly, making UDP suitable for streaming media, DNS queries, and VoIP where a retransmitted old packet is worse than a missed one.

**Network Devices**

Selecting the right device for a network scenario is a fundamental IT skill. A hub broadcasts every frame to all ports (Layer 1). A switch forwards frames only to the destination port using its MAC table (Layer 2). A router connects different IP networks and routes packets between them (Layer 3). A WAP (Wireless Access Point) bridges wireless 802.11 clients to the wired Ethernet network. A firewall inspects traffic and applies rules to allow or block packets based on address, port, and state.

Worked examples

Example 1: Tracing a web request through OSI layers: You type 'https://pgcc.edu' in a browser. Layer 7 (Application): HTTP GET request is generated. Layer 6 (Presentation): TLS encryption is applied. Layer 4 (Transport): TCP wraps the request in segments with port 443. Layer 3 (Network): IP headers are added with your IP and PGCC's IP. Layer 2 (Data Link): Ethernet frame is created with your MAC and the router's MAC. Layer 1 (Physical): bits are transmitted. At the destination, each layer strips its header (decapsulation) until the web server receives the original HTTP request.

Example 2: Diagnosing DHCP failure: A student's laptop shows 'Limited connectivity.' You check the IP address: it shows 169.254.1.47. This APIPA address (169.254.x.x range) immediately tells you the laptop could not reach a DHCP server. Diagnosis steps: (1) Check physical connection / Wi-Fi association. (2) Verify the DHCP server is running. (3) Release and renew: ipconfig /release then ipconfig /renew. (4) If a different device gets an IP normally, the problem is isolated to the first device — check NIC driver.

Common mistakes

Confusing Layer 2 (switch) with Layer 3 (router). Switches forward based on MAC addresses within one network. Routers connect different networks using IP addresses. A Layer 3 switch can perform routing functions but is still primarily a switching device.
Memorizing only that HTTPS=443 without understanding why. Port 443 is the well-known TCP port for TLS-encrypted HTTP. Knowing the protocol-to-port mapping enables you to configure firewalls and diagnose connectivity issues.
Thinking ARP and DNS do the same job. DNS translates domain names to IP addresses (Layer 7 → Layer 3). ARP translates IP addresses to MAC addresses (Layer 3 → Layer 2). They operate at different layers for different purposes.
Assuming a switch and hub are interchangeable. A hub broadcasts all traffic to all ports, creating collisions and consuming bandwidth. A switch learns MAC addresses and forwards frames only to the correct port — dramatically more efficient.
Forgetting that the default gateway must be on the same subnet as the device. If the gateway IP is on a different subnet from the device, the device cannot route packets to it. Gateway misconfiguration silently prevents all internet access.

Self-check

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

1. At which OSI layer do Ethernet switches operate?

Layer 1
Layer 2
Layer 3
Layer 4

2. What does a 169.254.x.x IP address indicate?

A valid static IP
A VPN-assigned address
DHCP failure — OS self-assigned APIPA
A public IP address

3. TCP's three-way handshake sequence is:

SYN → SYN-ACK → ACK
ACK → SYN → SYN-ACK
SYN → ACK → FIN
CONNECT → ACCEPT → CONFIRM

4. Which protocol automatically assigns IP address, subnet mask, gateway, and DNS to a client?

DNS
ARP
DHCP
ICMP

5. Which RFC 1918 range is NOT a private IP address range?

10.0.0.0/8
172.16.0.0/12
192.168.0.0/16
8.8.0.0/8

Self-check answers

1. B — Switches operate at Layer 2 (Data Link), forwarding frames based on MAC addresses.
2. C — APIPA (169.254.0.0/16) is self-assigned when the OS cannot reach a DHCP server.
3. A — The handshake always begins with the client's SYN, the server's SYN-ACK, and the client's final ACK.
4. C — DHCP (Dynamic Host Configuration Protocol) provides all four essential network configuration parameters.
5. D — 8.8.0.0/8 is publicly routable (assigned to Google). The three RFC 1918 private ranges are 10.x, 172.16–31.x, and 192.168.x.

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