Networking Fundamentals

Week of 2026-09-22 · Download .docx

Objectives

Identify and describe each layer of the OSI model and its function.
Compare the OSI model to the TCP/IP model and map layers between the two.
Explain the difference between IPv4 and IPv6 addressing, including bit length and notation.
Apply CIDR notation to calculate subnet size, network address, broadcast address, and usable host count.
Recall common well-known port numbers: HTTP, HTTPS, FTP, SSH, Telnet, SMTP, DNS, DHCP, and RDP.

Key terms

OSI model: Open Systems Interconnection model — a 7-layer framework describing how data travels across a network from physical medium to application.
Encapsulation: The process of wrapping data with protocol headers as it moves down the OSI stack toward the wire; each layer adds its own header.
TCP/IP model: A condensed 4-layer model (Network Access, Internet, Transport, Application) used in practice; maps to the OSI model.
IPv4: 32-bit internet addressing protocol written as four decimal octets (e.g., 192.168.1.1); approximately 4.3 billion unique addresses.
IPv6: 128-bit internet addressing protocol written as eight hexadecimal groups separated by colons; virtually unlimited address space.
CIDR: Classless Inter-Domain Routing — notation that expresses a network as an address plus a slash and the number of network bits (e.g., /24).
Subnet: A logical division of an IP network; hosts on the same subnet communicate directly without a router.
Default gateway: The IP address of the local router interface; packets destined for outside the local subnet are forwarded here.
ARP: Address Resolution Protocol — broadcasts a request to find which device on the local network owns a given IP address; the owner replies with its MAC address.
TCP: Transmission Control Protocol — a connection-oriented Layer 4 protocol providing reliable, ordered delivery using acknowledgments and retransmissions.
UDP: User Datagram Protocol — a connectionless Layer 4 protocol with no acknowledgment or retransmission; faster than TCP but unreliable.
TTL: Time To Live — a hop counter in IP packets decremented by each router; when it reaches zero the packet is dropped and an ICMP error is returned.

The concept

The OSI model is the single most important conceptual framework for the CompTIA Network+ exam. It divides network communication into seven numbered layers, each with a specific function. Understanding which protocols and devices operate at each layer is tested on every Network+ practice exam.

Layer 1, the Physical layer, handles raw bit transmission over cables, fiber, and radio waves. Layer 2, the Data Link layer, frames data for delivery within a single network segment using MAC addresses — switches operate here. Layer 3, the Network layer, handles IP addressing and routing between different networks — routers operate here. Layer 4, the Transport layer, manages end-to-end delivery using either TCP (reliable) or UDP (fast). Layers 5 through 7 (Session, Presentation, Application) handle session management, data format translation, and application-level services. In practice, the TCP/IP model condenses these into four layers: Network Access (Layers 1–2), Internet (Layer 3), Transport (Layer 4), and Application (Layers 5–7).

When data is sent across a network, each layer wraps the data from the layer above with its own header — a process called encapsulation. A web request starts as application data, gets a TCP header (Layer 4), an IP header (Layer 3), and finally an Ethernet frame header and trailer (Layer 2) before transmission. The receiving device strips each header in reverse order moving up the stack.

IPv4 uses 32-bit addresses written in dotted-decimal notation (for example, 192.168.10.1). The total address space is 2^32, approximately 4.3 billion addresses — a number that was exhausted globally by the early 2010s. IPv6 solves this with 128-bit addresses written as eight groups of four hexadecimal digits separated by colons, providing 2^128 unique addresses.

Subnetting uses CIDR notation to divide large address blocks into smaller subnets. The slash number tells you how many bits are the network portion; the remaining bits are for hosts. A /24 prefix leaves 8 host bits, giving 256 total addresses and 254 usable hosts (subtract the network address and broadcast address). A /26 leaves 6 host bits: 64 total, 62 usable. A /30 leaves 2 host bits: 4 total, 2 usable — common for point-to-point links.

Knowing common port numbers is a direct exam requirement. Key ports: SSH 22, Telnet 23, SMTP 25, DNS 53, DHCP 67/68, HTTP 80, HTTPS 443, FTP 20/21, RDP 3389. ARP resolves an IP address to a MAC address on the local segment; without ARP, Layer 3 routing cannot deliver a frame at Layer 2.

Worked examples

Example 1: Subnetting a /26: A technician is given the address block 192.168.10.0/26. Step 1 — the slash number is 26, so 26 bits are the network portion and 32 − 26 = 6 bits are the host portion. Step 2 — total addresses = 2^6 = 64. Usable hosts = 64 − 2 = 62 (subtract network address and broadcast). Step 3 — network address: 192.168.10.0. Broadcast address: 192.168.10.63 (all host bits set to 1). Step 4 — usable host range: 192.168.10.1 through 192.168.10.62. Repeat for /28: 4 host bits, 16 total, 14 usable; for /30: 2 host bits, 4 total, 2 usable.

Example 2: OSI layer identification drill: A company reports that users in the sales office cannot reach the internet but can reach each other. Identify the OSI layer involved. Users reaching each other means Layer 1 (Physical) and Layer 2 (Data Link) are working — the switch is delivering frames. The inability to reach the internet means Layer 3 (Network) is the fault layer — the router or default gateway is misconfigured or unreachable. Correct diagnosis: check the default gateway address with ipconfig, verify the router's WAN connection, and test reachability by pinging the default gateway IP. If ping to gateway fails, the fault is at Layer 3 on the local network; if it succeeds but internet is unreachable, the fault is upstream.

Common mistakes

Confusing the OSI and TCP/IP layer counts. The OSI model has 7 layers; the TCP/IP model has 4. On Network+ exam questions, 'which layer' always refers to the OSI model unless the question explicitly states TCP/IP.
Calculating subnet hosts as 2^n instead of 2^n − 2. Both the network address and the broadcast address are reserved and cannot be assigned to any device. Always subtract 2 from the total to get usable hosts.
Mixing up TCP and UDP use cases. UDP is not inferior — it is correct for DNS, VoIP, and streaming where speed matters more than completeness. TCP is correct for file transfer, web, and email where every byte must arrive. Neither is universally better.
Confusing ARP with DNS. ARP resolves an IP address to a MAC address on the local network segment (Layer 3 to Layer 2). DNS resolves a hostname to an IP address (application layer to Layer 3). They work at completely different layers and solve different problems.
Forgetting that IPv6 addresses are written in hexadecimal, not decimal. IPv4 uses four decimal octets. IPv6 uses eight hexadecimal groups. Reading 'ff02::1' as a decimal number is a common reading-comprehension error on subnetting questions.

Self-check

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

1. At which OSI layer does a router operate?

Layer 1 — Physical
Layer 2 — Data Link
Layer 3 — Network
Layer 4 — Transport

2. How many usable host addresses are available on a /28 subnet?

3. Which protocol automatically maps IP addresses to MAC addresses on a local network?

DNS
DHCP
ARP
ICMP

4. Which transport layer protocol would a streaming video application use to minimize latency?

TCP
FTP
UDP
SMTP

5. Which of the following is a private IPv4 address per RFC 1918?

8.8.8.8
10.0.0.1
172.217.0.1
1.1.1.1

Self-check answers

1. C — Routers operate at Layer 3 (Network layer) because they make forwarding decisions based on IP addresses, which are Layer 3 logical addresses.
2. A — A /28 leaves 4 host bits: 2^4 = 16 total addresses minus 2 (network + broadcast) = 14 usable hosts.
3. C — ARP (Address Resolution Protocol) broadcasts a request asking which device owns a given IP address; the owner replies with its MAC so frames can be delivered at Layer 2.
4. C — UDP's lack of handshake and retransmission overhead minimizes latency for real-time video. A small amount of packet loss causes minor visual artifacts, which is preferable to TCP retransmission stalls.
5. B — 10.0.0.0/8 is one of the three RFC 1918 private address ranges not routable on the public internet. The others are 172.16.0.0/12 and 192.168.0.0/16.

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