Networking and Operating Systems

Week of 2026-10-06 · Download .docx

Objectives

Name the seven OSI model layers in order and identify the protocol or device associated with each layer.
Explain the difference between TCP and UDP and identify appropriate use cases for each.
Describe DHCP and DNS and explain their roles in enabling network communication.
Convert between dotted-decimal subnet masks and CIDR prefix notation and calculate hosts per subnet.
Compare NTFS, FAT32, exFAT, and ext4 file systems and select the correct file system for common scenarios.

Key terms

OSI model: Open Systems Interconnection model — a 7-layer conceptual framework describing how data travels from application to physical medium and back.
IP address: A logical network address assigned to each device on a network; IPv4 = 32 bits (dotted-decimal), IPv6 = 128 bits (hexadecimal).
TCP: Transmission Control Protocol — a connection-oriented Layer 4 protocol that guarantees reliable, ordered delivery using acknowledgments and retransmission.
UDP: User Datagram Protocol — a connectionless Layer 4 protocol that sends datagrams with no acknowledgment or retransmission; faster but unreliable.
DHCP: Dynamic Host Configuration Protocol — a service that automatically leases IP addresses, subnet masks, default gateways, and DNS servers to network clients.
DNS: Domain Name System — a distributed directory that maps human-readable hostnames (www.example.com) to IP addresses.
Subnet mask: A 32-bit value that identifies which portion of an IP address is the network ID and which is the host ID. /24 = 255.255.255.0.
Default gateway: The IP address of the router interface on the local subnet — the exit point packets use to reach networks outside the local segment.
MAC address: A 48-bit hardware address burned into a NIC at the factory; written as six colon-separated hexadecimal pairs. Used for Layer 2 frame delivery.
NTFS: New Technology File System — the default Windows file system supporting file permissions (ACLs), encryption (EFS), journaling, and files over 4 GB.
FAT32: File Allocation Table 32-bit — a legacy file system limited to 4 GB per file and no permission support; widely compatible but not suitable for modern OS installations.
exFAT: Extended FAT — a file system for removable flash media (USB drives, SD cards) that removes FAT32's 4 GB file size limit and is natively readable on Windows and macOS.
ext4: Fourth Extended Filesystem — the default Linux file system supporting journaling, extents, POSIX permissions, and file sizes up to 16 TB.
Port number: A 16-bit number (0–65535) that identifies a specific application or service within a host. HTTP = 80, HTTPS = 443, FTP = 21, SSH = 22.

The concept

Networking and operating systems are the two disciplines that A+ Core 2 (220-1202) focuses on most heavily. This module connects the concepts that explain how computers communicate on a network and how software manages hardware resources.

**The OSI Model: A Communication Blueprint**

The OSI (Open Systems Interconnection) model breaks network communication into seven layers, each with a specific function. From bottom to top: Physical (Layer 1) handles bits on wire or radio; Data Link (Layer 2) handles MAC addressing and frame delivery within a single segment; Network (Layer 3) handles IP addressing and routing between networks; Transport (Layer 4) handles end-to-end delivery reliability (TCP) or speed (UDP); Session (Layer 5) manages sessions between applications; Presentation (Layer 6) handles data format and encryption; and Application (Layer 7) provides network services to user applications.

For the A+ exam, focus on Layers 1–4. Routers operate at Layer 3; switches at Layer 2; hubs at Layer 1. TCP and UDP live at Layer 4.

**TCP vs. UDP**

TCP (Transmission Control Protocol) establishes a connection before sending any data using the three-way handshake (SYN → SYN-ACK → ACK). It numbers each segment sequentially, requires the receiver to send acknowledgments, and retransmits any segment that is not acknowledged. This makes TCP reliable and ordered but adds latency overhead. Use TCP for web browsing (HTTP/HTTPS), email (SMTP/IMAP), and file transfer (FTP).

UDP (User Datagram Protocol) sends datagrams with no connection setup, no sequence numbers, and no acknowledgments. This makes UDP faster and appropriate for applications where a small amount of packet loss is acceptable: live video streaming, online gaming, DNS lookups, and VoIP.

**IP Addressing and Subnetting**

Every device on an IP network is assigned an IP address. IPv4 addresses are 32 bits written as four decimal octets (192.168.1.100). The subnet mask identifies which bits belong to the network (all 1s) and which belong to the host (all 0s). A /24 mask means 24 network bits, written as 255.255.255.0 — leaving 8 host bits and supporting 254 usable host addresses on the subnet.

DHCP automates address assignment. When a device connects, it broadcasts a DHCP Discover; the DHCP server (often the router) responds with an Offer; the client sends a Request; the server sends an Acknowledgment (DORA sequence). The device then uses the assigned IP, subnet mask, default gateway, and DNS server addresses.

DNS translates domain names to IP addresses through a hierarchy of authoritative name servers. When you type a URL, your device queries its configured DNS server, which recursively resolves the name to an IP address so a TCP connection can be established.

**File Systems**

Operating systems use file systems to organize data on storage media. Windows installs on NTFS because NTFS supports ACL permissions, EFS encryption, journaling (crash recovery), and large files. FAT32 is legacy and compatible with nearly every device but cannot hold files larger than 4 GB. exFAT extends FAT32 for modern flash drives by removing the file size limit. Linux uses ext4 by default, which provides journaling and full POSIX permission support necessary for a multi-user operating system.

Worked examples

Example 1: Subnetting a /24: A network administrator has the address block 10.0.1.0/24. How many usable host addresses are on this subnet? A /24 prefix leaves 8 bits for hosts (32 − 24 = 8). Total addresses = 2^8 = 256. Subtract 2: one for the network address (10.0.1.0) and one for the broadcast address (10.0.1.255). Usable hosts = 254. The first usable host is 10.0.1.1; the last is 10.0.1.254; the default gateway is typically assigned 10.0.1.1.

Example 2: Choosing a file system for a USB drive: A student needs to share a 6 GB video file between a Windows laptop and a MacBook. FAT32 is eliminated immediately — its maximum file size is 4 GB and the video is 6 GB. NTFS is readable on macOS but only as read-only by default without additional drivers. ext4 is unreadable on Windows or macOS without third-party tools. exFAT is natively readable and writable on both Windows (Vista SP1 and later) and macOS (10.6.5 and later) with no file size limit. Correct choice: format the USB drive as exFAT.

Common mistakes

Confusing IP addresses with MAC addresses. An IP address (Layer 3, 32-bit logical) is assigned by DHCP and can change. A MAC address (Layer 2, 48-bit hardware) is burned into the NIC and identifies the device on the local segment. ARP bridges the two: it maps a known IP to its MAC address.
Stating that TCP is always better than UDP. UDP is the correct choice for latency-sensitive applications where occasional packet loss is preferable to the overhead of acknowledgment and retransmission. DNS uses UDP because a retry is cheaper than a three-way handshake for a single short query.
Assuming all M.2 slots support NVMe — this also applies to file systems. Assume nothing: always verify the protocol (SATA vs. NVMe) and the file system label before drawing conclusions about compatibility or performance.
Mixing up port numbers 80 and 443. HTTP uses port 80 (unencrypted). HTTPS uses port 443 (TLS-encrypted). Firewalls that block port 443 will break secure web browsing. Firewalls that block port 80 break unencrypted HTTP but not HTTPS.
Forgetting that the /24 subnet supports 254 usable hosts, not 256. The network address (all host bits = 0) and the broadcast address (all host bits = 1) are reserved and cannot be assigned to a device. Always subtract 2 from the total address count to get usable hosts.

Self-check

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

1. At which OSI layer do routers operate?

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

2. Which protocol would be most appropriate for live video streaming?

TCP
FTP
UDP
HTTPS

3. A /24 subnet mask written in dotted-decimal is:

255.0.0.0
255.255.0.0
255.255.255.0
255.255.255.128

4. Which file system should be used for a USB drive that must be writable on both Windows and macOS without additional drivers?

NTFS
ext4
FAT32
exFAT

5. Which sequence describes the DHCP address assignment process?

Discover → Offer → Request → Acknowledge
Request → Discover → Offer → Acknowledge
Offer → Discover → Acknowledge → Request
Acknowledge → Request → Offer → Discover

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. C — UDP is used for live video streaming because its low overhead minimizes latency. Occasional packet loss causes minor visual artifacts, which is preferable to the stall caused by TCP retransmissions.
3. C — A /24 prefix means 24 consecutive 1-bits in the mask, which equals three fully set octets: 255.255.255.0.
4. D — exFAT is natively supported on both Windows and macOS with no file size limit, making it the correct choice for cross-platform removable drives with files over 4 GB.
5. A — DHCP uses the DORA sequence: Discover (client broadcasts), Offer (server responds with an IP), Request (client selects the offer), Acknowledge (server confirms the lease).

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