Network Layer
Network Layer
The Network Layer is the 3rd layer of the OSI model.
Its main job is to deliver data from source to destination across different networks.
Key Functions
- Logical addressing (IP address)
- Routing (path selection)
- Packet forwarding
- Internetworking
Real-life example: Choosing the best route on Google Maps to reach your destination.
Switching Techniques
Switching decides how data moves from sender to receiver through the network.
Circuit Switching
A dedicated communication path is established before data transmission and remains reserved until communication ends.
Steps
- Connection establishment
- Data transfer
- Connection termination
Characteristics
- Fixed path
- No packet loss
- Continuous transmission
Example: Traditional telephone call
Advantages
- Reliable
- No delay during transmission
Disadvantages
- Wastes bandwidth when not in use
- Not suitable for data networks
Packet Switching
Data is divided into small packets, and each packet may travel via different paths to the destination.
Characteristics
- No dedicated path
- Efficient bandwidth use
- Packets reassembled at destination
Example: Internet, email, WhatsApp messages
Advantages
- High efficiency
- Suitable for data communication
Disadvantages
- Packet delay
- Packet loss possible
Message Switching
Entire message is sent and stored at intermediate nodes before forwarding.
Characteristics
- No dedicated path
- Store-and-forward technique
Example: Old telegraph systems
Disadvantages
- High delay
- Not used in modern networks
Switching Techniques Comparison (Exam-Important)
| Feature | Circuit | Packet | Message |
|---|---|---|---|
| Path | Dedicated | Dynamic | Dynamic |
| Delay | Low | Variable | High |
| Efficiency | Low | High | Medium |
| Example | Telephone | Internet | Telegraph |
Logical Addressing
Logical addressing is used to identify devices uniquely in a network using IP addresses.
IPv4 Addressing
IPv4 uses 32-bit addresses, written in dotted decimal notation.
Example: 192.168.1.1
Total Addresses
2³² ≈ 4.3 billion
IPv4 Address Classes
| Class | Range | Network Bits | Host Bits | Use |
|---|---|---|---|---|
| A | 1 – 126 | 8 | 24 | Large networks |
| B | 128 – 191 | 16 | 16 | Medium networks |
| C | 192 – 223 | 24 | 8 | Small networks |
| D | 224 – 239 | – | – | Multicasting |
| E | 240 – 255 | – | – | Research |
Subnetting
Subnetting divides a large network into smaller sub-networks (subnets).
Purpose
- Better network management
- Reduced traffic
- Improved security
Subnet Mask Example : 255.255.255.0
Real-life example: Dividing a big company into department-wise networks.
IPv6 Addressing
IPv6 uses 128-bit addresses, written in hexadecimal format.
Example: 2001:0db8:85a3::8a2e:0370:7334
Advantages of IPv6
- Huge address space
- Better security
- Auto-configuration
- No NAT required
IPv4 vs IPv6 Comparison
| Feature | IPv4 | IPv6 |
|---|---|---|
| Address Size | 32-bit | 128-bit |
| Address Format | Decimal | Hexadecimal |
| Security | Optional | Built-in |
| Address Space | Limited | Very large |
Important Exam Questions
- Explain switching techniques with comparison
- Differentiate circuit, packet, and message switching
- Explain IPv4 addressing and classes
- What is subnetting? Explain with example
- Compare IPv4 and IPv6
Network Layer Protocols & Routing Techniques
Network Layer Protocols – Introduction
Network layer protocols help in:
- Address resolution
- Automatic IP configuration
- Communication between devices in a network
These protocols work along with IP to ensure correct data delivery.
ARP (Address Resolution Protocol)
ARP is used to find the MAC address of a device when its IP address is known.
IP → MAC address
Working of ARP
- Sender broadcasts ARP request: “Who has this IP?”
- Receiver replies with its MAC address
- Mapping is stored in ARP cache
Example: A computer knows the IP of another system in LAN but needs its MAC address to send data.
Key Points
- Used in IPv4
- Operates in LAN
- Essential for packet delivery
RARP (Reverse Address Resolution Protocol)
RARP is the reverse of ARP.
It finds IP address using MAC address.
MAC → IP address
Example: Diskless workstation requests IP address from server using its MAC address.
Limitations
- Needs RARP server
- Obsolete now
BOOTP (Bootstrap Protocol)
BOOTP assigns:
- IP address
- Gateway
- Boot file location
to a system during startup.
Features
- Static IP mapping
- Manual configuration required
Example: Used earlier for diskless machines to boot from network.
DHCP (Dynamic Host Configuration Protocol)
DHCP automatically assigns IP addresses and other network settings.
DHCP Process (DORA – Exam Important)
- Discover – Client broadcasts request
- Offer – Server offers IP
- Request – Client requests IP
- Acknowledge – Server confirms
Advantages
- No manual configuration
- Avoids IP conflicts
- Widely used today
Example: Mobile phone connecting to Wi-Fi gets IP automatically.
Comparison of Network Layer Protocols
| Protocol | Purpose | Status |
|---|---|---|
| ARP | IP → MAC | Active |
| RARP | MAC → IP | Obsolete |
| BOOTP | Boot & IP assignment | Rare |
| DHCP | Automatic IP assignment | Widely used |
Routing Techniques
Routing decides the best path for data packets to reach destination.
Intradomain Routing
Routing within a single organization or autonomous system (AS).
Types
- Distance Vector Routing: Uses hop count. Example: RIP
- Link State Routing: Uses complete network topology. Example: OSPF
Example: Routing inside a college campus network.
Interdomain Routing
Routing between different autonomous systems.
Protocol Used
-
BGP (Border Gateway Protocol)
Example: Routing between ISP networks on the Internet.
Intradomain vs Interdomain Routing (Exam Table)
| Feature | Intradomain | Interdomain |
|---|---|---|
| Area | Within AS | Between AS |
| Protocol | RIP, OSPF | BGP |
| Complexity | Low | High |
| Example | College network | Internet routing |
Important Exam Questions
- Explain ARP and RARP with examples
- Describe BOOTP and DHCP
- Explain DHCP process (DORA)
- Differentiate intradomain and interdomain routing
- Explain routing protocols with examples
Exam Tips
- Write DORA steps clearly
- Use comparison tables
- Draw simple diagrams
- Mention real-life examples
Conclusion
Network layer protocols like ARP, DHCP ensure correct addressing, while routing techniques ensure efficient packet delivery.
- DHCP is most widely used today
- Intradomain routing handles internal networks
- Interdomain routing forms the backbone of the Internet