Computer Network Network Layer and Its Protocol (ET: 1.5 Hours)
Network Layer and Its Protocol
    • 1
      Network Layer Functionalities
      Functionalities
      Logical Addressing
      • Assign unique IP addresses to devices.
      Routing
      • Determines best path for data to travel from source to destination.
      • Uses routing algorithms and routing tables.
      Packet Forwarding
      • Forwards packets based on their destination address, using routers to switch packets between different networks.
      Fragmentation & Reassembly
      • Handles fragmentation of packets if they are larger than the maximum transmission unit (MTU) of a network and reassembles them at the destination.
      Quality of Services
      • Prioritize certain types of traffic over others, ensuring that critical data receives the necessary bandwidth and resources
      Error Handling
      • Detect and handle errors during data transmission using protocols like ICMP (Internet Control Message Protocol).

    • 2
      IP Address
      IP Address stands for Internet Protocol Address. It is a logical address assigned to a device on a network so that the device can be identified and communicate with other devices.
      IP addresses are mainly divided into two versions:
      1. IPv4 (Internet Protocol Version 4)
      2. IPv6 (Internet Protocol Version 6)

    • 3
      IPv4 Address
      In a traditional classful interpretation, the network and host portions depend on the address class. In modern networking, the network/host boundary is normally determined by the subnet mask or CIDR prefix.
      IPv4: IPv4 uses a 32-bit addressing system. It is represented using four 8-bit octets in dotted-decimal notation.
      Example: 192.168.10.100
      Each octet can contain a value from 0 to 255.
      IPv4 Address Format:
      8 bits + 8 bits + 8 bits + 8 bits = 32 bits
      IPv4 provides approximately 232 = 4.29 billion possible addresses.
      Advantages of IPv4:
      • Widely supported by networking devices.
      • Simple and easy to understand.
      • Uses a 32-bit addressing system.
      • Well-supported by existing routing technologies.
      Disadvantages of IPv4:
      • Limited address space.
      • IPv4 addresses are insufficient for the continuously increasing number of devices.
      • Security was not built into the original IPv4 design in the same way as modern security mechanisms.

    • 4
      IPv6 Address
      IPv6: IPv6 stands for Internet Protocol Version 6. It was developed to overcome the address-space limitations of IPv4.
      IPv6 uses a 128-bit addressing system.
      Example: 2001:db8:1234:5678:0000:0000:0000:0001
      IPv6 addresses contain 8 groups, and each group represents 16 bits. Each group is written using hexadecimal digits from 0000 to FFFF.
      IPv6 Address Length:
      8 groups × 16 bits = 128 bits
      IPv6 provides approximately 2128 possible addresses, which is an extremely large address space.
      In a commonly used IPv6 global-unicast structure, the address may be divided into:
      • First 48 bits: Global Routing Prefix
      • Next 16 bits: Subnet ID
      • Last 64 bits: Interface ID
      Advantages of IPv6:
      • Provides a very large address space.
      • Supports a huge number of devices.
      • Provides better support for modern Internet-scale networks.
      • Supports efficient routing and address aggregation.
      • Includes features that support secure communication, such as IPsec.
      • Supports efficient multicast and autoconfiguration mechanisms.
      Disadvantages of IPv6:
      • IPv6 is not directly backward-compatible with IPv4.
      • Migration from IPv4 to IPv6 requires additional configuration and infrastructure.
      • Some older devices may not support IPv6.

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      Private and Public IP Address
      Private IP Address: A private IP address is used inside a local/private network such as a home, office, or institution. Private IPv4 addresses are not directly routable on the public Internet.
      Common Private IPv4 Ranges:
      • 10.0.0.0/8
      • 172.16.0.0/12
      • 192.168.0.0/16
      Example: A home Wi-Fi router may assign addresses such as 192.168.1.10 and 192.168.1.20 to connected devices.
      Public IP Address: A public IP address is used for communication over the Internet and is generally assigned by an Internet Service Provider (ISP).
      Private vs Public IP:
      FeaturePrivate IPPublic IP
      Used inLocal/private networkInternet/public network
      Internet routingNot directly routable on the public InternetRoutable on the Internet
      Example192.168.1.10ISP-assigned address
      Common useHome, office, institutionInternet communication
      Quick Comparison:
      FeatureIPv4IPv6
      Address length32 bits128 bits
      NotationDotted decimalHexadecimal, colon-separated
      Number of groups4 octets8 groups/hextets
      Value per group0–2550000–FFFF
      Address space2322128
      Example192.168.10.1002001:db8::1

    • 6
      What is NAT? Why we used it and how NAT translate?
      NAT (Network Address Translation) is a networking technique used in routers to translate private IP addresses into a public IP address before sending data to the Internet.
      It allows multiple devices in a private network to access the Internet using a single public IP address.
      Why NAT is Used
      • IP Address Conservation: NAT helps save public IPv4 addresses by allowing many devices to share one public IP.
      • Security: Internal private IP addresses are hidden from external networks, which increases network security.
      • Internet Access: It allows devices in a private network to communicate with external networks like the Internet.
      How NAT Translation Works
      • A device in a private network sends a request to the Internet using its private IP address.
      • The router replaces the private IP address with its own public IP address.
      • The request is sent to the destination server on the Internet.
      • When the response comes back, the router converts the public IP address back to the original private IP address and sends the data to the correct device.


    • 7
      Network Layer Protocol
      The Network Layer is the 3rd layer of the OSI model. Its main responsibility is to determine the best route for packets to travel from the source to the destination.
      The important protocols discussed under Network Layer are:
      1. IP (Internet Protocol)
      2. ARP (Address Resolution Protocol)
      3. ICMP (Internet Control Message Protocol)
      4. IGMP (Internet Group Management Protocol)

    • 8
      ARP (Address Resolution Protocol)
      ARP stands for Address Resolution Protocol.
      ARP is used in an IPv4 network to find the MAC address corresponding to a known IP address, mainly on the local network.
      IP address is a logical address, whereas a MAC address is a hardware/link-layer address associated with a network interface.
      Example:
      Suppose Device A knows that Device B has IP address 192.168.1.20, but Device A does not know Device B's MAC address.
      Step 1: ARP Request
      Device A broadcasts an ARP Request asking:
      "Who has IP address 192.168.1.20?"
      Step 2: ARP Reply
      The device having that IP address sends an ARP Reply containing its MAC address. The reply is normally sent directly to Device A.
      Step 3: Communication
      Device A now knows Device B's MAC address and can send the Ethernet frame to Device B.
      ARP Working:
      Known IP address → ARP Request → ARP Reply → MAC address obtained → Frame transmission
      Key Point: ARP resolves an IPv4 address to a MAC address on the local network.

    • 9
      ICMP (Internet Control Message Protocol)
      ICMP stands for Internet Control Message Protocol.
      ICMP is mainly used for error reporting, diagnostics, and control information related to IP communication.
      Some important ICMP messages are:
      Echo Request: Used to check whether a destination host is reachable. The ping command uses ICMP Echo Request.
      Echo Reply: Sent in response to an Echo Request when the destination responds successfully.
      Destination Unreachable: Indicates that a packet could not reach its intended destination.
      Time Exceeded: Indicates that a packet's allowed lifetime has expired. This message is also important in tools such as traceroute/tracert.
      Example:
      When a user runs ping 192.168.1.1, an ICMP Echo Request is sent to the destination. If the destination responds, an ICMP Echo Reply is received.
      Key Point: ICMP is used mainly for network error reporting and diagnostics; it does not provide reliable data delivery like TCP.

    • 10
      IGMP (Internet Group Management Protocol)
      IGMP stands for Internet Group Management Protocol.
      IGMP is used in IPv4 networks for multicast group management.
      In multicasting, one sender can transmit data to a group of receivers that have joined a particular multicast group.
      Example:
      Suppose a live cricket match is being streamed to many users. Users who want to receive the multicast stream can join the appropriate multicast group.
      Important IGMP Operations:
      Joining a Multicast Group:
      A host indicates that it wants to receive traffic for a particular multicast group.
      Maintaining Group Membership:
      Multicast routers use IGMP information to determine which multicast groups have active members on their connected networks.
      Leaving a Multicast Group:
      A host can indicate that it no longer wants to receive traffic from a multicast group.
      Key Point: IGMP is associated with IPv4 multicast group membership management.

    • 11
      Comparison of Network Layer Protocols
      ProtocolFull FormMain PurposeImportant Point
      IPInternet ProtocolLogical addressing and packet deliveryIPv4 = 32-bit, IPv6 = 128-bit
      ARPAddress Resolution ProtocolFind MAC address from an IPv4 addressUsed mainly on the local network
      ICMPInternet Control Message ProtocolError reporting and network diagnosticsUsed by ping
      IGMPInternet Group Management ProtocolIPv4 multicast group managementUsed for multicast membership

Topic Wise Question Bank
All Topics
    • ☆
      1
      Computer NetworkIP Address
      What is IP? Why does need IP in every network?
      Pubali, JHE, 23 |

      An IP (Internet Protocol) is a unique logical address assigned to every device connected to a network. It is used to identify devices and enable communication between them over a network or the Internet.

      Why Do We Need an IP Address in Every Network?

      An IP Address is essential because it allows devices to communicate and exchange data correctly within a network.

      • Uniquely identifies every device on the network.
      • Enables data packets to reach the correct destination.
      • Helps routers determine the best path for data transmission.
      • Supports communication between devices on local networks and the Internet.
      • Prevents data from being sent to the wrong device.

      Example

      When a computer sends a request to a web server, the IP Address identifies both the sender and the receiver, ensuring that the data reaches the correct destination.



    • ☆
      2
      Computer NetworkIP Address
      ২. (ক) IP address এবং MAC address-এর মাঝে তুলনা করুন।
      BPSC_MoHA_AP, 26 | Assistant Programmer (BPSC)

      An IP Address and a MAC Address are both used to identify devices in a computer network, but they work at different layers and have different purposes.

      IP Address: An IP Address is a logical address assigned to a device for identifying the device and determining its location within a network. It is mainly used for routing packets from one network to another.
      MAC Address: A MAC Address (Media Access Control Address) is a physical or hardware address associated with a network interface. It is mainly used for communication between devices within the same local network.

      Comparison Table:

      FeatureIP AddressMAC Address
      Full FormInternet Protocol AddressMedia Access Control Address
      TypeLogical AddressPhysical/Hardware Address
      OSI LayerNetwork Layer (Layer 3)Data Link Layer (Layer 2)
      PurposeUsed for logical addressing and routing.Used for identifying a network interface within a local network.
      FormatIPv4: 32-bit; IPv6: 128-bitUsually 48-bit (6 bytes)
      Example192.168.1.1000:1A:2B:3C:4D:5E
      ChangeabilityCan change depending on the network or configuration.Usually assigned to the network interface and is intended to be unique.
      Communication ScopeCan be used for communication across different networks.Primarily used for communication within the local network.


    • ☆
      3
      Computer NetworkIP Address

      ২(খ) Classful এবং Classless IP Address কী? এদের প্রধান পার্থক্যগুলো কী কী? নিচের IP-গুলোর Class নির্ণয় করুন।

      i) 00000001 00001011 00001011 11101111

      ii) 211.10.15.4

      BPSC_MoHA_AP, 26 | Assistant Programmer (BPSC)

      Classful IP Address: Classful IP Addressing হলো এমন একটি IP addressing পদ্ধতি যেখানে IPv4 address-কে পূর্বনির্ধারিত পাঁচটি class-এ ভাগ করা হয়: Class A, Class B, Class C, Class D এবং Class E। IP address-এর class অনুযায়ী network এবং host অংশ নির্ধারিত হয়।
      Classless IP Address: Classless IP Addressing, যা CIDR (Classless Inter-Domain Routing) নামেও পরিচিত, হলো এমন একটি IP addressing পদ্ধতি যেখানে network এবং host অংশের boundary নির্দিষ্ট class-এর উপর নির্ভর করে না। বরং subnet mask বা prefix length ব্যবহার করে network অংশ নির্ধারণ করা হয়।
      Classful ও Classless IP Address-এর প্রধান পার্থক্য:

      1. Address Division: Classful addressing-এ IP address পূর্বনির্ধারিত class অনুযায়ী ভাগ করা হয়, কিন্তু Classless addressing-এ prefix length অনুযায়ী ভাগ করা হয়।
      2. Subnet Mask: Classful addressing-এ default subnet mask ব্যবহার করা হয়, কিন্তু Classless addressing-এ বিভিন্ন subnet mask বা prefix length ব্যবহার করা যায়।
      3. Address Utilization: Classful addressing-এ অনেক IP address অপচয় হতে পারে, কিন্তু Classless addressing-এ IP address আরও দক্ষতার সাথে ব্যবহার করা যায়।
      4. Flexibility: Classful addressing কম flexible, কিন্তু Classless addressing বেশি flexible।
      5. CIDR Support: Classless addressing CIDR support করে, যা efficient IP allocation এবং route aggregation-এর জন্য ব্যবহৃত হয়।

      IP Class Determination:
      i) 00000001 00001011 00001011 11101111
      First Octet = 00000001
      The first bit of the first octet is 0.
      A Class A IP address always starts with the binary bit 0.
      Therefore, the given IP address belongs to Class A.
      Answer: Class A

      বাংলা:
      প্রথম Octet = 00000001
      প্রথম Octet-এর প্রথম bit হলো 0।
      Class A IP address-এর প্রথম bit সবসময় 0 হয়।
      অতএব, প্রদত্ত IP address-টি Class A।
      উত্তর: Class A

      ii) 211.10.15.4
      First Octet = 211
      The first octet range of Class C is 192–223.
      Since 211 falls within this range, the given IP address belongs to Class C.
      Answer: Class C

      বাংলা:
      প্রথম Octet = 211
      Class C-এর প্রথম Octet-এর range হলো 192–223।
      যেহেতু 211 এই range-এর মধ্যে রয়েছে, তাই প্রদত্ত IP address-টি Class C।
      উত্তর: Class C

      Final Answer:
      i) 00000001 00001011 00001011 11101111 → Class A
      ii) 211.10.15.4 → Class C


    • ☆
      4
      Computer NetworkIP Address
      What is IP address? Explain the necessity of IP address in network?
      Pubali, HE, 23 | Bank

      IP Address

      An IP address (Internet Protocol address) is a unique numerical identifier assigned to each device connected to a network. It is used to identify devices and enable communication between them over a network or the Internet.

      Necessity of IP Address in a Network

      An IP address uniquely identifies each device in a network, ensuring that data reaches the correct destination.

      It helps in identifying the source and destination of data packets during communication.

      Routers use IP addresses to determine the best path for forwarding data across different networks.

      IP addresses enable devices to access network resources such as servers, printers, and the Internet.

      Without IP addresses, communication between devices in a network would not be possible.

      Example

      When a computer requests a webpage, its IP address is used by the web server to send the requested data back to the correct computer.



    • ☆
      5
      Computer NetworkIP Address
      What is IPv6? Why IPv6 needed?
      SPCBL, SAME, 22 | AME/ANE/AE

      IPv6 (Internet Protocol version 6) is the latest version of the Internet Protocol used to identify devices on a network. It uses a 128-bit address format, allowing a very large number of unique IP addresses.

      Why IPv6 is Needed

      • IPv4 address space is almost exhausted.
      • IPv6 provides a much larger address space.
      • Improves security with built-in IPsec support.
      • Supports better routing and network efficiency.


    • ☆
      6
      Computer NetworkIP Address
      Why does IPv6 header architecture faster processing than IPv4 architecture.
      RAKUB, ANSE, 26 | AME/ANE/AE

      IPv6 header architecture enables faster processing than IPv4 due to several design simplifications that reduce router workload at each hop.

      • Fixed header size: IPv6 uses a fixed 40-byte header. IPv4 has a variable header length (20 to 60 bytes) because of optional fields, forcing routers to parse and calculate the actual header size every time.
      • No header checksum: IPv6 removes the header checksum entirely. IPv4 routers must recalculate and verify the checksum at every hop, adding CPU overhead. IPv6 delegates error detection to upper-layer protocols like TCP and UDP.
      • No router fragmentation: IPv4 allows routers to fragment packets when they are too large, which requires complex reassembly logic. IPv6 only permits fragmentation by the source host, so routers never perform this task.
      • Simplified header fields: IPv6 has fewer header fields (8 vs 14 in IPv4), making parsing faster. Fields like Header Length, Identification, Flags, Fragment Offset, and Options are removed or relocated to extension headers.
      • Extension headers: Optional features in IPv6 are placed in separate extension headers that routers can skip unless specifically needed, unlike IPv4 where options are embedded in the main header.

      Comparison Table:

      FeatureIPv4IPv6
      Header sizeVariable (20–60 bytes)Fixed 40 bytes
      Header checksumYes, recalculated at every hopRemoved
      FragmentationRouters can fragmentOnly source fragments
      Header fields14 fields8 fields
      Options handlingEmbedded in main headerExtension headers

Topic Wise Question Bank
All Topics
    • ☆
      1
      Computer NetworkNAT Concept
      What is NAT? Why we used it and how NAT translate?
      CB, AE(IT)/AHME/SO, 21 | AME/ANE/AE

      NAT (Network Address Translation) is a networking technique used in routers to translate private IP addresses into a public IP address before sending data to the Internet.

      It allows multiple devices in a private network to access the Internet using a single public IP address.

      Why NAT is Used

      • IP Address Conservation: NAT helps save public IPv4 addresses by allowing many devices to share one public IP.
      • Security: Internal private IP addresses are hidden from external networks, which increases network security.
      • Internet Access: It allows devices in a private network to communicate with external networks like the Internet.

      How NAT Translation Works

      • A device in a private network sends a request to the Internet using its private IP address.
      • The router replaces the private IP address with its own public IP address.
      • The request is sent to the destination server on the Internet.
      • When the response comes back, the router converts the public IP address back to the original private IP address and sends the data to the correct device.


    • ☆
      2
      Computer NetworkNAT Concept
      Network Address Translation (NAT) maps internal networks to the public internet.
      (a) Explain the historical IP addressing limitation that made NAT a necessity globally.
      (b) Explain the step-by-step logical translation process that occurs at a branch router when an internal employee (IP 192.168.1.5) sends a web request to an external server, and how the router correctly handles the returning response packet.
      Combined Bank, O(IT-23), 26 | Officer (IT)

      Historical Limitation Behind NAT

      NAT became necessary because of the limited number of available IPv4 addresses.

      IPv4 uses a 32-bit addressing system, which can provide approximately 4.3 billion unique IP addresses. During the early development of the Internet, this number seemed sufficient. However, as the number of computers, smartphones, servers, IoT devices, and internet users increased rapidly worldwide, IPv4 addresses started running out.

      Public IP addresses must be globally unique, but there were not enough addresses for every device connected to the Internet.

      To solve this problem, private IP addressing and NAT were introduced.

      Using NAT:

      • Many internal devices can share a single public IP address.
      • Private IP addresses can be reused in different organizations.
      • IPv4 address exhaustion is reduced significantly.

      Thus, NAT became an important solution for conserving public IPv4 addresses and enabling large-scale Internet growth.

      (b) Step-by-Step NAT Translation Process

      Scenario:

      • Internal Employee IP = 192.168.1.5
      • Branch Router Public IP = 203.0.113.10
      • External Web Server IP = 8.8.8.8

      Step 1: Employee Sends Request

      The employee's computer creates a web request packet.

      Source IP = 192.168.1.5
      Destination IP = 8.8.8.8

      Since 192.168.1.5 is a private IP address, it cannot travel directly over the Internet.

      Step 2: Packet Reaches the Branch Router

      The router receives the packet and checks its NAT table.

      The router replaces the private source IP address with its own public IP address.

      For example:

      Before NAT:
      Source = 192.168.1.5
      Destination = 8.8.8.8

      After NAT:
      Source = 203.0.113.10
      Destination = 8.8.8.8

      The router stores this mapping in its NAT translation table.

      Example NAT Table Entry:

      Private IPPublic IP
      192.168.1.5203.0.113.10

      Step 3: Packet Travels Across the Internet

      The modified packet is sent through the Internet to the external web server.

      The web server sees the request coming from the router’s public IP address instead of the private internal address.

      Step 4: External Server Sends Response

      The web server replies to:

      Destination IP = 203.0.113.10

      The response packet travels back to the branch router.

      Step 5: Router Performs Reverse Translation

      The router checks the NAT table and finds the matching internal device.

      It replaces the destination public IP address with the original private IP address.

      Before Reverse NAT:
      Destination = 203.0.113.10

      After Reverse NAT:
      Destination = 192.168.1.5

      Step 6: Response Delivered to Employee

      The router forwards the packet to the employee’s computer.

      The employee receives the web response correctly even though the internal private IP address was hidden from the Internet.

      NAT allows multiple internal devices using private IP addresses to communicate with external networks using a smaller number of public IP addresses. It conserves IPv4 addresses and provides an additional layer of network privacy.



1. What is an IP Address? Explain why an IP address is necessary in a computer network.

2. Compare IP Address and MAC Address based on their type, OSI layer, purpose, format, and communication scope.

3. What is IPv4? Explain its 32-bit addressing structure with a suitable example.

4. What is IPv6? Why is IPv6 needed? Explain the major advantages of IPv6 over IPv4.

5. Compare IPv4 and IPv6 based on address length, notation, address space, number of groups, and major features.

6. What are Private IP Address and Public IP Address? Explain their differences with suitable examples.

7. Why does IPv6 header architecture allow faster packet processing than IPv4 header architecture? Explain the major changes that improve processing efficiency.

8. Explain the role of IP, ARP, ICMP, and IGMP in computer networking.

📘 Network Layer and it Protocol
1. Which layer of the OSI model is responsible for logical addressing and routing?
2. What is the main function of the Network Layer?
3. What type of address is an IP address?
4. Which device primarily operates at the Network Layer?
5. Which of the following is a major functionality of the Network Layer?
6. What is routing?
7. What is packet forwarding?
8. What does MTU stand for?
9. What is fragmentation in the Network Layer?
10. What does IP stand for?
11. What is the length of an IPv4 address?
12. How many octets are present in an IPv4 address?
13. Which notation is normally used for IPv4 addresses?
14. What is the valid range of values for each IPv4 octet?
15. Which of the following is a valid IPv4 address?
16. How many possible addresses are provided by a 32-bit IPv4 address space?
17. What does IPv6 stand for?
18. How many bits are used in an IPv6 address?
19. How many groups or hextets are normally present in a full IPv6 address?
20. Which notation is used for IPv6 addresses?
21. Which of the following is a valid IPv6 address?
22. What is the main reason IPv6 was developed?
23. Which IPv6 field helps identify packets belonging to a particular traffic flow?
24. Which of the following is an advantage of IPv6?
25. Which protocol resolves a known IPv4 address to a MAC address on a local network?
26. What does ARP stand for?
27. What type of ARP message is normally broadcast on the local network?
28. Which protocol is mainly used for network error reporting and diagnostics?
29. Which command commonly uses ICMP Echo Request and Echo Reply messages?
30. Which ICMP message indicates that a packet could not reach its intended destination?
31. Which ICMP message is associated with an expired packet lifetime and is important to traceroute/tracert?
32. What is the primary purpose of IGMP?
33. Which type of communication allows one sender to transmit data to a group of receivers?
34. Which Network Layer protocol is associated with IPv4 multicast group membership?
35. Which protocol is mainly responsible for logical addressing and packet delivery?
36. Which of the following is NOT a primary function of the Network Layer?

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