Networking: Practice Problems

Networking — Practice Problems

12 practice problems covering fundamental networking concepts: OSI model, TCP/IP, DNS, HTTP, subnetting, and network security.

Worked Examples

Example 1: Subnetting, CIDR Calculation

Problem: Given the IP block 192.168.1.0/24, divide it into 4 equal subnets. List each subnet’s address range, broadcast address, and number of usable hosts.

Solution:

Step 1: Determine the new prefix length

Original: /24 (256 addresses) Need 4 subnets: $2^2 = 4$ , so we need 2 additional bits

New prefix: /26 (64 addresses per subnet)

Step 2: Calculate subnet addresses

Subnet	Network Address	Usable Range	Broadcast	Hosts
1	192.168.1.0/26	192.168.1.1 - 192.168.1.62	192.168.1.63	62
2	192.168.1.64/26	192.168.1.65 - 192.168.1.126	192.168.1.127	62
3	192.168.1.128/26	192.168.1.129 - 192.168.1.190	192.168.1.191	62
4	192.168.1.192/26	192.168.1.193 - 192.168.1.254	192.168.1.255	62

Key insight: Each subnet has $2^{32-26} - 2 = 62$ usable addresses (subtract network and broadcast addresses). The block size is $256/4 = 64$ .

Example 2: TCP Three-Way Handshake

Problem: Trace the TCP connection establishment between a client (10.0.0.1:5000) and server (10.0.0.2:80). What flags are set in each step?

Solution:

Step 1: SYN (Client → Server)

Source: 10.0.0.1:5000
Destination: 10.0.0.2:80
Flags: SYN=1
Sequence: 1000

Step 2: SYN-ACK (Server → Client)

Source: 10.0.0.2:80
Destination: 10.0.0.1:5000
Flags: SYN=1, ACK=1
Sequence: 5000
Acknowledgment: 1001

Step 3: ACK (Client → Server)

Source: 10.0.0.1:5000
Destination: 10.0.0.2:80
Flags: ACK=1
Sequence: 1001
Acknowledgment: 5001

Key insight: The three-way handshake ensures both sides agree on initial sequence numbers and establishes a full-duplex connection. The SYN flag synchronizes sequence numbers, and the ACK flag acknowledges the previous segment.

Example 3: DNS Resolution Chain

Problem: Trace the DNS resolution process when a user types “www.example.com” in a browser, starting from a cold cache.

Solution:

Step 1: Browser checks local cache → miss

Step 2: OS checks /etc/hosts → miss

Step 3: OS queries configured DNS resolver (e.g., 8.8.8.8)

Step 4: Recursive resolver checks its cache → miss

Step 5: Resolver queries root DNS servers (.) → referral to .com TLD servers

Step 6: Resolver queries .com TLD servers → referral to example.com authoritative servers

Step 7: Resolver queries example.com authoritative server → returns A record (93.184.216.34)

Step 8: Recursive resolver caches the result (TTL-based)

Step 9: Resolver returns 93.184.216.34 to client

Step 10: Browser initiates TCP connection to 93.184.216.34:443

Key insight: DNS is hierarchical and distributed. The recursive resolver does the heavy lifting, caching results to reduce load on authoritative servers. TTL (Time To Live) controls how long records are cached.

OSI Model and Protocols

Subnetting and Addressing

TCP/IP and HTTP

Network Security and Infrastructure

Topic	Site	Link
Security	Infrastructure	Security
Linux Networking	Infrastructure	Linux
Databases	Infrastructure	Databases
Computer Networks	University	Computer Networks

Intuition

Networking is about connecting systems and communicating data: From Ethernet cables to the Internet, networking protocols define how data travels between devices. Understanding the protocol stack (TCP/IP model) is essential for troubleshooting and design.

Why it matters: Every internet-connected application depends on networking. Understanding protocols, addressing, and routing is essential for IT professionals and developers.

The key insight: The layered model (physical, data link, network, transport, application) means each layer only needs to interface with its neighbours, changes at one layer do not affect others.

Common Mistakes

Confusing OSI layers and their responsibilities: The 7 OSI layers (Physical, Data Link, Network, Transport, Session, Presentation, Application) each have distinct roles. A common error is placing encryption at the Transport layer (it belongs at Presentation or Application), or assuming IP handles reliability (that is TCP’s job at Transport).

Using classful subnetting when CIDR is standard: Classful addressing (Class A/B/C with fixed masks) is obsolete. Modern networks use CIDR (Classless Inter-Domain Routing) with variable-length subnet masks. Confusing /24 with a Class C network leads to incorrect subnet calculations. Always work with CIDR notation.

Forgetting that DNS resolves names to IPs, not the reverse: DNS maps domain names to IP addresses. The reverse (IP to name) uses reverse DNS (rDNS/PTR records). Students sometimes claim “DNS finds the website for you”, more precisely, DNS translates the name so routing can find the IP.

Networking: Practice Problems