Networking Basics Interview Questions and Answers for Placements

Last Updated: March 2026

Introduction to Computer Networks

Computer networking forms the backbone of modern computing, enabling communication between devices across the globe. Understanding networking fundamentals is essential for software engineers, as virtually every application today communicates over a network. From TCP/IP protocols to HTTP/HTTPS, DNS, and security - networking knowledge is tested in virtually every technical interview.

Why Networking is Essential for Placements

Universal Requirement: Every application uses network communication
System Design Foundation: Can't design scalable systems without networking knowledge
DevOps/SRE Roles: Network troubleshooting is core skill
Security Focus: Network security critical in modern applications
Cloud Native: Microservices communicate via networks

Key Concepts and Theory

OSI Model vs TCP/IP Model

OSI Model (7 Layers):

Layer	Name	Function	Examples
7	Application	User interface, network apps	HTTP, FTP, SMTP, DNS
6	Presentation	Data format, encryption	SSL/TLS, JPEG, ASCII
5	Session	Session management	NetBIOS, PPTP
4	Transport	End-to-end delivery	TCP, UDP
3	Network	Routing, logical addressing	IP, ICMP, OSPF
2	Data Link	Physical addressing, framing	Ethernet, MAC, PPP
1	Physical	Bits on wire	Cables, Hubs, Signals

TCP/IP Model (4 Layers):

Layer	OSI Equivalent	Protocols
Application	5-7	HTTP, FTP, SMTP, DNS
Transport	4	TCP, UDP
Internet	3	IP, ICMP, ARP
Network Access	1-2	Ethernet, Wi-Fi

Mnemonic for OSI: "Please Do Not Throw Sausage Pizza Away"

TCP vs UDP

Feature	TCP	UDP
Connection	Connection-oriented	Connectionless
Reliability	Reliable (acknowledgments)	Unreliable
Order	Ordered delivery	No ordering
Speed	Slower (overhead)	Fast
Use Case	Web, Email, File Transfer	Streaming, Gaming, DNS
Header Size	20-60 bytes	8 bytes
Flow Control	Yes	No
Congestion Control	Yes	No

TCP Header:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|          Source Port          |       Destination Port        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                        Sequence Number                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                    Acknowledgment Number                      |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Data |           |U|A|P|R|S|F|                               |
| Offset| Reserved  |R|C|S|S|Y|I|            Window             |
|       |           |G|K|H|T|N|N|                               |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           Checksum            |         Urgent Pointer        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Practice Questions with Solutions

Question 1: Explain TCP 3-Way Handshake. ⭐⭐ [Medium]

The 3-way handshake establishes a TCP connection between client and server.

Process:

Client                              Server
   |                                   |
   |-------- SYN (seq=x) ----------->|  Step 1: Client sends SYN
   |                                   |        "I want to connect"
   |                                   |
   |<-- SYN-ACK (seq=y, ack=x+1) ----|  Step 2: Server responds
   |                                   |        "I got it, you there?"
   |                                   |
   |-------- ACK (ack=y+1) --------->|  Step 3: Client confirms
   |                                   |        "I'm here, let's talk"
   |                                   |
   |<========= DATA TRANSFER =========>|

Why 3-way?

SYN: Client sends initial sequence number (ISN)
SYN-ACK: Server acknowledges and sends its ISN
ACK: Client acknowledges server's ISN
Ensures both sides can send and receive

Sequence Numbers:

Protect against delayed duplicate packets
Provide reliability through acknowledgment

Python Socket Example:

# Server
import socket
server = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
server.bind(('localhost', 8080))
server.listen(5)
conn, addr = server.accept()  # Completes 3-way handshake

# Client
import socket
client = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
client.connect(('localhost', 8080))  # Initiates 3-way handshake

Question 2: What is the Difference between HTTP and HTTPS? ⭐ [Easy]

Feature	HTTP	HTTPS
Security	Unencrypted	Encrypted with SSL/TLS
Port	80	443
Certificate	Not required	SSL certificate required
Performance	Faster	Slower (encryption overhead)
URL	http://	https://
SEO	Lower ranking	Preferred by search engines

HTTPS Handshake (TLS 1.2):

1. Client Hello
   - Supported TLS versions
   - Cipher suites supported
   - Random bytes (client_random)

2. Server Hello
   - Chosen TLS version
   - Chosen cipher suite
   - Random bytes (server_random)
   - SSL Certificate (public key)

3. Client Verification
   - Verify certificate (CA, expiry, domain)
   - Generate pre-master secret
   - Encrypt with server's public key

4. Key Generation
   - Both generate session keys from:
     client_random + server_random + pre-master

5. Finished
   - Exchange encrypted "finished" messages
   - Secure channel established

Why HTTPS matters:

Confidentiality: Data encrypted in transit
Integrity: Data can't be modified
Authentication: Verify server identity
Trust: Padlock icon builds user confidence

Question 3: Explain DNS Resolution Process. ⭐⭐ [Medium]

DNS (Domain Name System) translates human-readable domain names to IP addresses.

Resolution Process:

User types: www.example.com

1. Browser Cache
   - Check if recently resolved
   
2. OS Cache (hosts file, DNS client)
   - Check local DNS cache
   
3. Router Cache
   - Check router's DNS cache
   
4. ISP DNS Server (Recursive Resolver)
   - If not cached, start recursive query
   
5. Root Name Server
   - "I don't know, ask .com servers"
   - Returns TLD server address
   
6. TLD Server (.com)
   - "I don't know, ask example.com servers"
   - Returns authoritative server
   
7. Authoritative DNS Server
   - "www.example.com is at 93.184.216.34"
   
8. Return to Client
   - Cache the result (TTL duration)
   - Connect to IP address

DNS Record Types:

Record	Purpose	Example
A	IPv4 address	example.com → 93.184.216.34
AAAA	IPv6 address	example.com → 2606:2800:220:1:248:1893:25c8:1946
CNAME	Canonical name (alias)	www.example.com → example.com
MX	Mail exchange	example.com → mail.example.com
NS	Name server	example.com → ns1.example.com
TXT	Text record	SPF, DKIM verification
SOA	Start of authority	Zone information

DNS Caching:

Browser cache: Few minutes
OS cache: Configurable (usually hours)
TTL in DNS record: Set by domain owner

Question 4: What is the Difference between GET and POST? ⭐ [Easy]

Feature	GET	POST
Purpose	Retrieve data	Submit data
Visibility	Data in URL	Data in body
Caching	Can be cached	Not cached
Bookmarkable	Yes	No
Length Limit	Limited by URL (2048 chars)	Unlimited
Security	Not for sensitive data	Safer for sensitive data
Idempotent	Yes	No
History	Stored in browser history	Not stored

GET Request:

GET /users?id=123 HTTP/1.1
Host: api.example.com

POST Request:

POST /users HTTP/1.1
Host: api.example.com
Content-Type: application/json

{
    "name": "John",
    "email": "john@example.com"
}

When to Use:

GET: Search, filter, retrieve data, bookmarkable pages
POST: Create resources, submit forms, sensitive data

Other HTTP Methods:

PUT: Update entire resource (idempotent)
PATCH: Partial update
DELETE: Remove resource
HEAD: Get headers only
OPTIONS: Get supported methods

Question 5: Explain TCP Congestion Control. ⭐⭐⭐ [Hard]

Congestion Control prevents network overload by regulating data flow.

Key Mechanisms:

1. Slow Start:

Start with small congestion window (cwnd = 1 MSS)
Double cwnd every RTT (exponential growth)
Continues until threshold (ssthresh) or packet loss

2. Congestion Avoidance:

After reaching ssthresh, linear growth
Add 1 MSS per RTT
More conservative than slow start

3. Fast Retransmit:

On 3 duplicate ACKs, retransmit immediately
Don't wait for timeout

4. Fast Recovery:

After fast retransmit, don't go to slow start
Set cwnd = ssthresh + 3 MSS
Continue in congestion avoidance

TCP Tahoe vs Reno:

Tahoe: On loss, cwnd = 1, slow start
Reno: Fast recovery, better performance

Modern Algorithms:

CUBIC: Used in Linux default
BBR: Google's bottleneck bandwidth approach

Visualization:

Congestion Window
    │
    │    ╱‾‾‾‾‾‾‾‾‾‾‾‾‾‾╲
    │   ╱                  ╲___ Slow Start
    │  ╱                        then Congestion Avoidance
    │ ╱     Packet Loss → Drop
    │╱                        Fast Recovery
    └────────────────────────────── Time

Question 6: What is a Load Balancer? Explain Types. ⭐⭐⭐ [Hard]

Load Balancer distributes incoming network traffic across multiple servers to ensure high availability and reliability.

Why Load Balancing:

High availability (no single point of failure)
Scalability (add/remove servers)
Better performance (distribute load)
Health monitoring (remove failed servers)

Types of Load Balancers:

1. Layer 4 (Transport Layer):

Based on IP address and port
Faster, less processing
Doesn't inspect content
Examples: HAProxy (L4 mode), AWS NLB

2. Layer 7 (Application Layer):

Based on HTTP headers, URL, cookies
Content-aware routing
SSL termination
Examples: Nginx, AWS ALB, HAProxy (L7 mode)

Load Balancing Algorithms:

Algorithm	Description	Use Case
Round Robin	Sequential distribution	Equal capacity servers
Least Connections	To server with fewest connections	Variable request duration
Least Response Time	To fastest responding server	Performance critical
IP Hash	Based on client IP	Session persistence
Weighted	Based on server capacity	Different server specs

Session Persistence (Sticky Sessions):

Route same client to same server
Methods: IP hash, cookies, session ID

Health Checks:

Load Balancer → Server A: /health (OK)
Load Balancer → Server B: /health (FAIL)
                    ↓
            Remove B from pool

Question 7: Explain REST API Principles. ⭐⭐ [Medium]

REST (Representational State Transfer) is an architectural style for designing networked applications.

Six Constraints:

1. Client-Server:

Separation of concerns
Client handles UI, server handles data
Independent evolution

2. Stateless:

No client context stored on server
Each request contains all necessary information
Server doesn't remember previous requests

3. Cacheable:

Responses must define themselves as cacheable
Improves scalability and performance

4. Uniform Interface:

Resource identification (URLs)
Resource manipulation through representations
Self-descriptive messages
Hypermedia as engine of application state (HATEOAS)

5. Layered System:

Client can't tell if connected directly to server or intermediary
Load balancers, caches, proxies transparent

6. Code on Demand (Optional):

Server can extend client functionality (JavaScript)

RESTful URL Design:

GET    /users          # List all users
GET    /users/123      # Get user 123
POST   /users          # Create new user
PUT    /users/123      # Update user 123 (full)
PATCH  /users/123      # Partial update user 123
DELETE /users/123      # Delete user 123
GET    /users/123/orders  # Get user's orders

HTTP Status Codes:

Code	Meaning	Example
200	OK	Successful GET, PUT
201	Created	Successful POST
204	No Content	Successful DELETE
400	Bad Request	Invalid input
401	Unauthorized	Missing auth
403	Forbidden	No permission
404	Not Found	Resource doesn't exist
500	Server Error	Exception occurred

Question 8: What is CORS and How to Handle It? ⭐⭐ [Medium]

CORS (Cross-Origin Resource Sharing) is a security mechanism that restricts web pages from making requests to a different domain than the one serving the web page.

Same-Origin Policy:

Protocol + Host + Port must match
http://example.com:80 vs https://example.com:443 → Different!

CORS Headers:

Request Headers:

Origin: http://example.com
Access-Control-Request-Method: POST
Access-Control-Request-Headers: X-Custom-Header

Response Headers:

Access-Control-Allow-Origin: http://example.com
Access-Control-Allow-Methods: GET, POST, PUT
Access-Control-Allow-Headers: Content-Type, X-Custom-Header
Access-Control-Allow-Credentials: true
Access-Control-Max-Age: 86400

Types of CORS Requests:

1. Simple Requests:

GET, HEAD, POST
Standard headers only
Content-Type: application/x-www-form-urlencoded, multipart/form-data, text/plain
No preflight

2. Preflight Requests:

Non-simple requests trigger OPTIONS preflight
Browser checks if actual request is safe
Server must respond to OPTIONS

Preflight Flow:

Browser → OPTIONS /api/data
          Origin: http://example.com
          Access-Control-Request-Method: DELETE
          
Server ← 204 No Content
         Access-Control-Allow-Origin: http://example.com
         Access-Control-Allow-Methods: GET, POST, DELETE

Browser → DELETE /api/data (actual request)

Handling CORS:

Server-side (Express.js):

const cors = require('cors');
app.use(cors({
    origin: 'http://example.com',
    methods: ['GET', 'POST'],
    credentials: true
}));

Proxy (Development):

// React development server proxy
"proxy": "http://localhost:3001"

Never use * with credentials!

Question 9: Explain WebSockets and Difference from HTTP. ⭐⭐⭐ [Hard]

WebSocket provides full-duplex communication over a single TCP connection.

HTTP vs WebSocket:

Feature	HTTP	WebSocket
Communication	Half-duplex	Full-duplex
Connection	Short-lived	Persistent
Overhead	High (headers every request)	Low (after handshake)
Server Push	Not supported (polling needed)	Native support
Real-time	Polling/WebSocket needed	Native
Protocol	http:// or https://	ws:// or wss://

WebSocket Handshake:

Client Request:
GET /chat HTTP/1.1
Host: example.com
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Sec-WebSocket-Version: 13

Server Response:
HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=

Python WebSocket Example:

# Server (using websockets library)
import asyncio
import websockets

async def handler(websocket, path):
    async for message in websocket:
        print(f"Received: {message}")
        await websocket.send(f"Echo: {message}")

start_server = websockets.serve(handler, "localhost", 8765)
asyncio.get_event_loop().run_until_complete(start_server)
asyncio.get_event_loop().run_forever()

# Client
import asyncio
import websockets

async def client():
    uri = "ws://localhost:8765"
    async with websockets.connect(uri) as websocket:
        await websocket.send("Hello!")
        response = await websocket.recv()
        print(response)

asyncio.get_event_loop().run_until_complete(client())

Use Cases:

Real-time chat
Live notifications
Gaming
Stock tickers
Collaborative editing

Alternatives:

SSE (Server-Sent Events): One-way server push over HTTP
Long Polling: HTTP workaround for server push
WebRTC: Peer-to-peer, good for video/voice

Question 10: What is a CDN and How Does It Work? ⭐⭐ [Medium]

CDN (Content Delivery Network) is a distributed network of servers that delivers content to users based on their geographic location.

How CDN Works:

Without CDN:                    With CDN:
                                
User in India                   User in India
    │                               │
    │ Request                       │ Request
    ▼                               ▼
Server in US                  Edge Server in Mumbai
(200ms latency)               (20ms latency)
    │                               │
    │ Response                      │ Cached Response
    │ (Slow)                        │ (Fast)
    ▼                               ▼
   User                           User

CDN Benefits:

Reduced Latency: Serve from nearest edge server
Lower Bandwidth Costs: Origin serves less data
High Availability: Multiple servers, automatic failover
DDoS Protection: Absorb and distribute attack traffic
SSL/TLS Optimization: Terminate SSL at edge

CDN Caching:

First Request:
User → Edge Server → Origin Server
                       ↓
                Cache content at edge
                
Subsequent Requests:
User → Edge Server (cached)
         ↓
    Serve directly (faster)

Cache Invalidation:

TTL (Time To Live): Auto-expire after set time
Purge: Manually clear cache
Versioning: Change filename (style.v1.css → style.v2.css)

Popular CDNs:

Cloudflare
AWS CloudFront
Google Cloud CDN
Akamai
Fastly

CDN for Dynamic Content:

Route optimization (not caching)
TCP connection reuse
Keep-alive with origin
Smart routing (avoid congested paths)

Common Interview Follow-Up Questions

"What happens when you type google.com in browser?" → DNS → TCP handshake → TLS → HTTP request → Server processing → Response → Rendering
"Difference between router and switch?" → Router (Layer 3, between networks), Switch (Layer 2, within network)
"What is NAT?" → Network Address Translation, maps private IPs to public IP
"How does traceroute work?" → Uses TTL increment, ICMP time exceeded or port unreachable
"Explain SYN flood attack?" → Attacker sends SYN without completing handshake, exhausting server resources

Companies Asking Networking Questions

Company	Frequency	Focus Areas
Amazon	Very High	Load balancing, AWS networking
Google	Very High	Protocol design, Performance
Cloudflare	Very High	DNS, CDN, Security
Akamai	Very High	CDN, Edge computing
Cisco	Very High	Routing, Switching, Protocols
Netflix	High	CDN, Video streaming
Uber	High	Microservices networking

Preparation Tips

Understand Packet Flow: From application to wire and back
Practice Wireshark: See real protocols in action
Learn Cloud Networking: VPCs, subnets, security groups
Know Trade-offs: TCP vs UDP, REST vs gRPC
Security Focus: HTTPS, TLS, certificates
Scalability: Load balancing, CDNs, caching
Troubleshooting: ping, traceroute, netstat, curl

FAQ

Q: What's the difference between a hub, switch, and router?
A: Hub (Layer 1, broadcasts to all), Switch (Layer 2, MAC-based forwarding), Router (Layer 3, IP-based forwarding between networks).

Q: Explain ARP (Address Resolution Protocol).
A: Maps IP addresses to MAC addresses on local network. "Who has IP 192.168.1.1? Tell MAC address."

Q: What is MTU and why does it matter?
A: Maximum Transmission Unit. Largest packet size. If exceeded, fragmentation occurs (performance impact).

Q: Difference between symmetric and asymmetric encryption?
A: Symmetric uses same key for encrypt/decrypt (fast). Asymmetric uses public/private key pair (secure key exchange).

Q: What is a reverse proxy?
A: Sits in front of servers, forwarding client requests. Used for load balancing, SSL termination, caching.

Strong networking knowledge is the foundation of distributed systems! 🌐

Networking Basics Questions Placement