Top 10 High-Frequency Questions

1. Which layer of the OSI model handles physical addressing, node-to-node data transfer, and operates using MAC addresses and switches?

That’s the Data Link Layer (Layer 2).

Its job is node-to-node (device-to-device) delivery on the same local network. It frames the data, handles error detection, and uses physical addressing — the MAC address burned into each network interface.

OSI Model
────────────────────────────
Layer 7  Application
Layer 6  Presentation
Layer 5  Session
Layer 4  Transport
Layer 3  Network       → IP
Layer 2  Data Link     → MAC  ← HERE
Layer 1  Physical      → Bits
────────────────────────────

Why switches belong here: a switch is a Layer 2 device. It reads MAC addresses and forwards frames to the specific port where the destination device lives, rather than broadcasting everywhere.

PC A                    SWITCH                    PC B
MAC: AA:AA        ┌────────────────┐        MAC: BB:BB
      ───────────→ │  MAC Table    │ ───────────→
                   │                │
                   │ AA → Port 1    │
                   │ BB → Port 2    │
                   └────────────────┘

Layer	Name	Address type
3	Network	IP
2	Data Link	MAC
1	Physical	None (bits)

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2. What is the primary operational difference between a Layer 2 Switch and a Layer 1 Hub?

The difference is intelligence.

• Hub (Layer 1) — blindly broadcasts incoming data to every port. No understanding of addresses, just electrical signal repetition.
• Switch (Layer 2) — learns which MAC address lives on which port, builds a MAC address table, and forwards frames only to the intended destination.

HUB
──────────────

             HUB
              │
      ┌───────┼───────┐
      ↓       ↓       ↓
     PC A    PC B    PC C

If A sends to B:

     PC A
       │
       ↓
      HUB
     /   \
    ↓     ↓
   PC B  PC C

Both B and C receive the signal.

SWITCH
──────────────

             SWITCH
          ┌────┼────┐
          ↓    ↓    ↓
         PC A PC B  PC C

If A sends to B:

     PC A
       │
       ↓
     SWITCH
       │
       ↓
     PC B

Only the required port receives the frame.

The consequences:

• A hub creates one big collision domain — traffic from every port can collide.
• A switch creates a separate collision domain per port and keeps traffic isolated.

Example: if 4 devices on a hub talk, all hear everything. On a switch, only the target device receives its frames.

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3. Which transmission protocol should be selected for an application where speed and minimal overhead are critical, and occasional data loss is acceptable?

UDP (User Datagram Protocol).

UDP is a connectionless, best-effort protocol. It has almost no overhead:

• No connection setup (no handshake).
• No acknowledgments.
• No retransmission of lost packets.
• No ordering guarantees.

TCP

Client                         Server
  │                              │
  │──────── SYN ────────────────→│
  │←────── SYN-ACK ──────────────│
  │──────── ACK ────────────────→│
  │                              │
  │──────── DATA ───────────────→│
  │←──────── ACK ────────────────│
  │                              │
  │   Lost packet?               │
  │←──── retransmission ─────────│

UDP

Client                         Server
  │                              │
  │──────── DATA ───────────────→│
  │──────── DATA ───────────────→│
  │──────── DATA ───────────────→│
  │                              │

No handshake
No ACK
No retransmission

That’s exactly what you want when speed beats reliability — video streaming, live gaming, VoIP, DNS lookups. A dropped frame in a video call is a blip; the next frame arrives anyway.

Compare with TCP: reliable and ordered, but it pays for that with handshakes, acks, and retransmissions — latency and overhead.

---

4. During a TCP Three-Way Handshake, what is the second packet sent to establish the connection?

The second packet is SYN-ACK.

The full sequence:

Client                              Server
  │                                   │
  │────────── SYN ──────────────────→│
  │       "I want to connect"         │
  │                                   │
  │←──────── SYN + ACK ──────────────│
  │       "I agree + acknowledge"     │
  │                                   │
  │────────── ACK ──────────────────→│
  │       "Confirmed"                 │
  │                                   │
  │        CONNECTION ESTABLISHED     │

So:

1. SYN
     ↓
2. SYN-ACK   ← Answer
     ↓
3. ACK
     ↓
Connection established

After the third step, both directions are synchronized and data can flow.

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5. Which of the following is a key advantage of IPv6 over IPv4?

The big one is the 128-bit address space — a virtually infinite pool that removes the need for NAT.

IPv4
────
32 bits
   ↓
~4.3 billion addresses

IPv6
────
128 bits
   ↓
~3.4 × 10³⁸ addresses
   ↓
Massive address space

Why it matters:

• IPv4 — 32-bit addresses (~4.3 billion). We ran out, which is why NAT exists to share one public IP among many devices.
• IPv6 — 128-bit addresses. Every device can have its own globally unique address, so NAT becomes unnecessary.

IPv4 Network

             Internet
                 │
          Public IPv4
                 │
                NAT
                 │
       ┌─────────┼─────────┐
       ↓         ↓         ↓
     PC 1       PC 2     Phone
 192.168.1.2 192.168.1.3 192.168.1.4

Bonus advantages:

• Built-in IPSec — security integrated by design.
• SLAAC — stateless address autoconfiguration; devices can assign their own addresses without a DHCP server.

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6. What is the primary role of the Domain Name System (DNS) on the Internet?

DNS is the phonebook of the internet — it translates human-readable names like google.com into the numeric IP addresses computers actually use.

You type:

google.com
    │
    ↓
   DNS
    │
    ↓
142.250.72.14
    │
    ↓
Browser connects to server

Without DNS:

You
 │
 └── "google.com"
          │
          ↓
       Computer
          │
          └── "I need an IP address!"

With DNS:

Browser
   │
   │ "What is google.com?"
   ↓
DNS Resolver
   │
   ↓
DNS Server
   │
   ↓
142.250.72.14
   │
   ↓
Browser connects

How it works: your device asks a resolver, which queries a hierarchy of DNS servers:

Client
  │
  ↓
DNS Resolver
  │
  ↓
Root DNS
  │
  ↓
TLD DNS (.com)
  │
  ↓
Authoritative DNS
  │
  ↓
IP Address

Each returns the answer or points to the next server that knows it. Results are cached to keep it fast.

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7. How does a Virtual Private Network (VPN) protect user data over a public network?

A VPN uses two techniques together: tunneling and encryption.

1. Tunneling — your data packet is wrapped (encapsulated) inside another packet addressed to the VPN server. This creates a tunnel between your device and the VPN server.
2. Encryption — the encapsulated data is scrambled with strong cryptographic protocols, so even if it’s intercepted, it’s unreadable.

Your Device
     │
     │  Original data
     ↓
  Encrypt
     │
     ↓
┌──────────────────────────┐
│      VPN Tunnel           │
│   🔒 Encrypted Traffic    │
└──────────────────────────┘
     │
     ↓
Public Internet
     │
     ↓
 VPN Server
     │
     ↓
Decrypt + Unwrap
     │
     ↓
Destination

To an outside observer, they see encrypted traffic to a VPN endpoint — not your actual activity. That’s how a VPN protects data on public Wi-Fi and hides traffic from the ISP.

---

8. What makes HTTPS secure compared to standard HTTP traffic?

HTTPS encrypts the data with SSL/TLS, while HTTP sends everything as plain text.

• HTTP — transmits data in clear text over port 80. Anyone on the path (or network) can read it.
• HTTPS — wraps the traffic in an SSL/TLS cryptographic layer over port 443. Data is unreadable to snoopers.

HTTP
────

Client
  │
  │ login=alice&password=123
  ↓
Internet
  │
  ↓
Server

Data travels in plain text.

HTTPS
─────

Client
  │
  │ 🔒 TLS Encryption
  ↓
[ 9F2A...Encrypted Data... ]
  │
  ↓
Internet
  │
  ↓
Server
  │
  ↓
Decrypt

What TLS provides:

                 TLS
                  │
        ┌─────────┼─────────┐
        ↓         ↓         ↓
Confidentiality Integrity Authentication
        │         │         │
      Secret    Detects    Verifies
      data      tampering  real server

• Confidentiality — encryption keeps content secret.
• Integrity — tampering is detected.
• Authentication — the site’s certificate proves you’re talking to the real server.

HTTP:   [login: alice, password: hunter2]
        ↓
        Visible as plain text

HTTPS:  [0x9F2A...encrypted blob...]
        ↓
        Unreadable to snoopers

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9. What is the functional purpose of a Subnet Mask in network configuration?

A subnet mask splits an IP address into a Network ID and a Host ID.

IP:         192.168.10.45
Mask:       255.255.255.0

            Network       Host
            <-------->    <->
IP:         192.168.10    .45
Mask:       255.255.255   .0

Therefore:

Network ID: 192.168.10.0
Host ID:              45

With /24:

IP:   192.168.10.45
      └──────┬──────┘
       Network  Host
       24 bits 8 bits

In binary:

Subnet Mask:

11111111.11111111.11111111.00000000
<--------- Network --------><-Host->
             24 bits          8 bits

Why it matters:

• Routers use the Network ID to decide where to send traffic — same network = deliver locally, different network = route elsewhere.
• It defines how big a subnet is (how many hosts fit).

Example: with 255.255.255.0, the first 24 bits are the network and the last 8 bits identify the host.

2⁸ = 256 total addresses

256
 ├── 1 Network address
 ├── 1 Broadcast address
 └── 254 Usable host addresses

So up to 254 usable host addresses are available in that subnet.

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10. What is the primary purpose of an IP address in computer networking?

An IP address identifies and locates a device on a network so it can communicate.

Sender
  │
  │ Destination IP:
  │ 192.168.1.50
  ↓
Router
  │
  ↓
Device
IP: 192.168.1.50

Two roles:

• Identification — identifies the destination device/interface.
• Locating — the address helps routers determine where the device is and deliver data to it.

Like a mailing address: it tells the network where the destination is so the packet can be delivered.

              IP Address
                   │
          ┌────────┴────────┐
          ↓                 ↓
   Identification        Location
          │                 │
   "Which device?"    "Which network?"

An IP address is logical — unlike the MAC address, it can change and is managed by the network layer.

MAC Address
     ↓
Layer 2
     ↓
Local network delivery
     ↓
"Which network interface?"

        vs.

IP Address
     ↓
Layer 3
     ↓
Routing between networks
     ↓
"Which network/location?"

Example:

             Router
           /         \
          /           \
   Network A         Network B
       │                 │
       ↓                 ↓
      PC A              PC B

   MAC → local delivery
   IP  → routing between networks