1. Models & Layers

OSI Model (7 layers → AP.ST.NDP)
👉 Application → Presentation → Session → Transport → Network → Data Link → Physical

TCP/IP Model (4 layers → AT.IN)
👉 Application → Transport → Internet → Network Access

2. Layers & Functions

1️⃣ Application Layer (OSI + TCP/IP)

What it does: User interaction, data generation, services.
Functions:

Provides services to the user
Runs software applications
File transfer, email, browsing

Protocols:
HTTP/HTTPS, FTP, SMTP, POP3, IMAP, DNS, SNMP

Devices used:

Application servers
Web servers, Mail servers, DNS servers
End-user devices (PC, mobile)

2️⃣ Presentation Layer (OSI)

Data formatting, encryption, compression
Functions:

Encrypt / Decrypt
Compress / Decompress
Convert data formats (JPEG, PDF, MP3)

Devices used:

No physical device (software layer)
Used inside apps for encryption (e.g., SSL/TLS libraries)

3️⃣ Session Layer (OSI)

Start, manage, end communication (sessions)
Functions:

Login session control
Keep connection alive
Synchronization

Devices used:

No device (software-based)
Used in session management tools & authentication systems

4️⃣ Transport Layer (OSI + TCP/IP)

Reliable/Unreliable delivery, segmentation, flow control.

Protocols:

TCP → Reliable, connection-oriented
UDP → Fast, no guarantee

Ports Examples:
80 (HTTP), 443 (HTTPS), 21 (FTP), 25 (SMTP), 110 (POP3)

Devices used:

Firewalls
Load balancers
Routers (for port forwarding)

5️⃣ Network Layer (OSI) / Internet Layer (TCP/IP)

Logical addressing & routing

Functions:

Assign IP addresses
Route packets between networks

Protocols:
IPv4, IPv6, ICMP, ARP, RARP, IGMP

🌍 IP Address & Routing

Devices:

Router
Layer 3 Switch
Firewall (partly)

6️⃣ Data Link Layer (OSI) / Network Access Layer (TCP/IP)

MAC addressing, framing, error detection

Protocols:
Ethernet, PPP, Frame Relay, HDLC

🔐 MAC Address Layer → Local communication

Devices:

Switch (Layer 2 switch)
Bridge
Access Points (Wi-Fi AP)
NIC (Network Card)

7️⃣ Physical Layer (OSI)

Raw bits, electrical signals, physical connection

Functions:

Cables, connectors
Data transmission (0s and 1s)

🔌 Electricity & Hardware → Cables and Basic Devices

Devices:

Cables (Ethernet, Fiber, Coax)
Hub
Repeater
Connectors / Patch Panels

👉 Memory: “Physical = Things you can touch directly.”

3. Common Protocols & Ports

Protocol	Port
FTP	20, 21
SSH	22
Telnet	23
SMTP	25
DNS	53
HTTP	80
HTTPS	443
SNMP –Simple Network Management Protocol is a tool used to monitor and manage network devices like routers, switches, and servers.	161
DHCP – Dynamic Host Configuration Protocol automatically gives IP address to devices in a network.	67, 68

🔑 Devices to Remember

📘 OSI Layers & Devices — Simplest Table

Layer No.	Layer Name	Devices
L1	Physical	Hub, Repeater, Cable, Modem – (HR-CM)
L2	Data Link	Switch, Bridge, NIC, Access Point – (SB-NA)
L3	Network	Router
L4	Transport	Firewall, Load Balancer
L5	Session	No Device
L6	Presentation	No Device
L7	Application	Gateway, Proxy, Application Firewall

🎯 One-Line Quick Memory

L1 Hub
L2 Switch
L3 Router
L4 Firewall
L5 -
L6 -
L7 Gateway

🚀 MCQs ON NETWORK LAYERS, FUNCTIONS & PROTOCOLS

⭐ SECTION 1: OSI MODEL & LAYER-WISE FUNCTIONS

1. OSI model has how many layers?

A. 5
B. 6
C. 7
D. 8
✔ Ans: C

2. Which OSI layer provides end-to-end communication?

A. Network
B. Transport
C. Data Link
D. Session
✔ Ans: B

3. Which layer is responsible for routing?

A. Data Link layer
B. Network layer
C. Physical layer
D. Transport layer
✔ Ans: B

4. Which OSI layer is responsible for error detection and MAC addressing?

A. Data Link layer
B. Network layer
C. Session layer
D. Application layer
✔ Ans: A

5. Logical addressing (IP address) works at which layer?

A. Layer 1
B. Layer 2
C. Layer 3
D. Layer 4
✔ Ans: C

6. Physical addressing (MAC address) is at which layer?

A. Network
B. Data Link
C. Application
D. Transport
✔ Ans: B

7. The layer responsible for encryption and decryption is:

A. Session
B. Transport
C. Presentation
D. Application
✔ Ans: C

8. Which layer manages dialog control and sessions?

A. Presentation
B. Session
C. Data Link
D. Transport
✔ Ans: B

9. The topmost layer of OSI model is:

A. Transport
B. Presentation
C. Application
D. Network
✔ Ans: C

10. Data transmission format (syntax, compression) is handled by:

A. Application layer
B. Presentation layer
C. Data link layer
D. Physical layer
✔ Ans: B

⭐ SECTION 2: TCP/IP MODEL

11. TCP/IP model has ——— layers.

A. 5
B. 4
C. 6
D. 7
✔ Ans: B

12. Which layer does the Internet layer in TCP/IP correspond to?

A. Data Link
B. Network
C. Transport
D. Session
✔ Ans: B

13. SMTP works at which TCP/IP layer?

A. Application
B. Transport
C. Internet
D. Network Access
✔ Ans: A

14. The Transport layer protocols in TCP/IP are:

A. IP and ICMP
B. TCP and UDP
C. HTTP and FTP
D. ARP and RARP
✔ Ans: B

15. The TCP/IP layer responsible for framing and physical addressing:

A. Internet
B. Transport
C. Application
D. Network Access
✔ Ans: D

⭐ SECTION 3: IMPORTANT PROTOCOLS & THEIR LAYERS

16. HTTP, FTP, DNS belong to which layer?

A. Application
B. Transport
C. Network
D. Data Link
✔ Ans: A

17. Which protocol is connection-oriented?

A. UDP
B. TCP
C. IP
D. ARP
✔ Ans: B

18. Which protocol resolves IP address to MAC address?

A. DNS
B. DHCP
C. ARP
D. ICMP
✔ Ans: C – ARP is a protocol that finds the MAC address of a device when you only know its IP address.

19. ICMP is used for:

A. Email transfer
B. Error reporting and diagnostics
C. File transfer
D. Name resolution
✔ Ans: B – ICMP is a protocol used to send error messages and test network connectivity (like Ping).

20. Ping command uses:

A. HTTP
B. UDP
C. ICMP
D. RARP
✔ Ans: C

21. SMTP is used for:

A. Receiving emails
B. Sending emails
C. File transfer
D. Remote login
✔ Ans: B

22. POP3 and IMAP are used for:

A. Sending mail
B. Remote login
C. Receiving mail
D. DNS lookup
✔ Ans: C

23. FTP uses which ports?

A. 20 & 21
B. 23
C. 53
D. 80
✔ Ans: A

24. Telnet works on port:

A. 23
B. 21
C. 80
D. 53
✔ Ans: A

25. DNS uses which port?

A. 20
B. 23
C. 53
D. 110
✔ Ans: C

26. HTTPS uses port:

A. 80
B. 443
C. 21
D. 110
✔ Ans: B

⭐ SECTION 4: DATA LINK LAYER DETAILS

27. Switch operates at which layer?

A. Layer 1
B. Layer 2
C. Layer 3
D. Layer 7
✔ Ans: B

28. Router operates at:

A. Layer 2
B. Layer 3
C. Layer 4
D. Layer 7
✔ Ans: B

29. Frame Check Sequence (FCS) is used for:

A. Encryption
B. Error detection
C. Routing
D. Key exchange
✔ Ans: B – FCS is a value added at the end of a frame to detect errors during data transmission.

30. PPP and HDLC are protocols of:

A. Physical layer
B. Data Link layer
C. Network layer
D. Transport layer
✔ Ans: B – PPP (Point-to-Point Protocol) is a WAN protocol used to connect two devices directly using serial links. HDLC (High-Level Data Link Control) is a WAN protocol used for reliable, bit-oriented communication over point-to-point links.

⭐ SECTION 5: IP ADDRESSING & NETWORK LAYER

31. IP is:

A. Connection-oriented
B. Connection-less
C. Reliable
D. Error-correcting
✔ Ans: B

32. Which protocol assigns IP addresses dynamically?

A. DNS
B. DHCP
C. FTP
D. ICMP
✔ Ans: B

33. Which protocol performs packet fragmentation?

A. ARP
B. IP
C. TCP
D. DHCP
✔ Ans: B

34. IPv6 address size is:

A. 32 bits
B. 64 bits
C. 128 bits
D. 256 bits
✔ Ans: C

35. Packet switching is performed at:

A. Application
B. Network
C. Data link
D. Transport
✔ Ans: B

⭐ SECTION 6: TRANSPORT LAYER

36. TCP provides:

A. Flow control
B. Congestion control
C. Reliability
D. All of the above
✔ Ans: D

37. UDP is used for:

A. File transfer
B. Video streaming
C. Secure communication
D. Email
✔ Ans: B

38. Port number size is:

A. 8 bits
B. 16 bits
C. 32 bits
D. 64 bits
✔ Ans: B

39. Segmentation occurs at:

A. Network layer
B. Transport layer
C. Data link layer
D. Physical layer
✔ Ans: B

40. Flow control is implemented using:

A. Checksum
B. Sliding window
C. TTL
D. Hop count
✔ Ans: B

⭐ SECTION 7: APPLICATION LAYER

41. Which protocol is used for remote login?

A. FTP
B. IMAP
C. Telnet
D. SMTP
✔ Ans: C

42. Which is a directory service protocol?

A. SMTP
B. LDAP
C. ICMP
D. POP
✔ Ans: B

43. Which is NOT an Application layer protocol?

A. HTTP
B. FTP
C. SMTP
D. ARP
✔ Ans: D

44. SNMP is used for:

A. Name resolution
B. Remote device monitoring
C. Email transfer
D. Routing
✔ Ans: B

45. DHCP uses: UDP or TCP?

A. TCP
B. UDP
C. Both
D. None
✔ Ans: B

⭐ SECTION 8: PHYSICAL LAYER

46. Which layer converts data into signals?

A. Transport
B. Presentation
C. Network
D. Physical
✔ Ans: D

47. Bit synchronization is done at:

A. Data link
B. Physical layer
C. Network layer
D. Session layer
✔ Ans: B

48. Hub operates at:

A. Layer 1
B. Layer 2
C. Layer 3
D. Layer 7
✔ Ans: A

⭐ SECTION 9: INTERNET PROTOCOL SUITE

49. Which protocol is used for network troubleshooting?

A. SNMP
B. ICMP
C. FTP
D. SMTP
✔ Ans: B – ICMP is a protocol used to send error messages and check network connectivity (like Ping).

50. Path MTU discovery uses:

A. TCP
B. ICMP
C. HTTPS
D. ARP
✔ Ans: B – Path MTU Discovery is a process that finds the largest packet size that can travel across a network path without breaking (fragmentation).

⭐ SECTION 10: MOST-EXPECTED Q

51. Three-way handshake is used by:

A. UDP
B. TCP
C. ICMP
D. DHCP
✔ Ans: B

52. Which layer ensures that packets reach the destination in order?

A. Data Link
B. Network
C. Transport
D. Session
✔ Ans: C

53. Which layer is responsible for QoS?

A. Network
B. Application
C. Transport
D. Data Link
✔ Ans: A

54. Which protocol resolves domain names to IP addresses?

A. FTP
B. DNS
C. DHCP
D. SNMP
✔ Ans: B

55. Which of the following is a routing protocol?

A. RIP
B. SMTP
C. FTP
D. DHCP
✔ Ans: A – RIP (Routing Information Protocol) is a simple distance-vector routing protocol that chooses paths based on hop count.

Network Layers, functions, and protocols

1. Models & Layers

2. Layers & Functions

1️⃣ Application Layer (OSI + TCP/IP)

2️⃣ Presentation Layer (OSI)

3️⃣ Session Layer (OSI)

4️⃣ Transport Layer (OSI + TCP/IP)

5️⃣ Network Layer (OSI) / Internet Layer (TCP/IP)

6️⃣ Data Link Layer (OSI) / Network Access Layer (TCP/IP)

7️⃣ Physical Layer (OSI)

3. Common Protocols & Ports

🔑 Devices to Remember

📘 OSI Layers & Devices — Simplest Table

🎯 One-Line Quick Memory

🚀 MCQs ON NETWORK LAYERS, FUNCTIONS & PROTOCOLS

⭐ SECTION 1: OSI MODEL & LAYER-WISE FUNCTIONS

1. OSI model has how many layers?

2. Which OSI layer provides end-to-end communication?

3. Which layer is responsible for routing?

4. Which OSI layer is responsible for error detection and MAC addressing?

5. Logical addressing (IP address) works at which layer?

6. Physical addressing (MAC address) is at which layer?

7. The layer responsible for encryption and decryption is:

8. Which layer manages dialog control and sessions?

9. The topmost layer of OSI model is:

10. Data transmission format (syntax, compression) is handled by:

⭐ SECTION 2: TCP/IP MODEL

11. TCP/IP model has ——— layers.

12. Which layer does the Internet layer in TCP/IP correspond to?

13. SMTP works at which TCP/IP layer?

14. The Transport layer protocols in TCP/IP are:

15. The TCP/IP layer responsible for framing and physical addressing:

⭐ SECTION 3: IMPORTANT PROTOCOLS & THEIR LAYERS

16. HTTP, FTP, DNS belong to which layer?

17. Which protocol is connection-oriented?

18. Which protocol resolves IP address to MAC address?

19. ICMP is used for:

20. Ping command uses:

21. SMTP is used for:

22. POP3 and IMAP are used for:

23. FTP uses which ports?

24. Telnet works on port:

25. DNS uses which port?

26. HTTPS uses port:

⭐ SECTION 4: DATA LINK LAYER DETAILS

27. Switch operates at which layer?

28. Router operates at:

29. Frame Check Sequence (FCS) is used for:

30. PPP and HDLC are protocols of:

⭐ SECTION 5: IP ADDRESSING & NETWORK LAYER

31. IP is:

32. Which protocol assigns IP addresses dynamically?

33. Which protocol performs packet fragmentation?

34. IPv6 address size is:

35. Packet switching is performed at:

⭐ SECTION 6: TRANSPORT LAYER

36. TCP provides:

37. UDP is used for:

38. Port number size is:

39. Segmentation occurs at:

40. Flow control is implemented using:

⭐ SECTION 7: APPLICATION LAYER

41. Which protocol is used for remote login?

42. Which is a directory service protocol?

43. Which is NOT an Application layer protocol?

44. SNMP is used for:

45. DHCP uses: UDP or TCP?

⭐ SECTION 8: PHYSICAL LAYER

46. Which layer converts data into signals?

47. Bit synchronization is done at:

48. Hub operates at:

⭐ SECTION 9: INTERNET PROTOCOL SUITE

49. Which protocol is used for network troubleshooting?

50. Path MTU discovery uses:

⭐ SECTION 10: MOST-EXPECTED Q

51. Three-way handshake is used by:

52. Which layer ensures that packets reach the destination in order?

53. Which layer is responsible for QoS?

54. Which protocol resolves domain names to IP addresses?

55. Which of the following is a routing protocol?