Network Layer

Network Layer

Network Layer provides routing and forwarding of packets across networks.

IP Addressing

IPv4 Address

IPv4 address is 32-bit (4 bytes), written as dotted decimal.

Format: A.B.C.D (each 0-255)

Example: 192.168.1.1

IP Address Classes

Class A:

• Range: 1.0.0.0 to 126.255.255.255
• Network bits: 8
• Host bits: 24
• Subnet mask: 255.0.0.0

Class B:

• Range: 128.0.0.0 to 191.255.255.255
• Network bits: 16
• Host bits: 16
• Subnet mask: 255.255.0.0

Class C:

• Range: 192.0.0.0 to 223.255.255.255
• Network bits: 24
• Host bits: 8
• Subnet mask: 255.255.255.0

Class D: Multicast (224.0.0.0 to 239.255.255.255)

Class E: Reserved (240.0.0.0 to 255.255.255.255)

Subnetting

Subnetting divides network into smaller subnets.

CIDR Notation: IP/prefix_length

Example: 192.168.1.0/24

• Network: 192.168.1.0
• Subnet mask: 255.255.255.0
• Hosts: 2^8 - 2 = 254 (excluding network and broadcast)

Subnet Calculation:

• Network address: IP AND subnet mask
• Broadcast address: Network address OR inverted subnet mask
• First host: Network address + 1
• Last host: Broadcast address - 1

Special IP Addresses

Loopback: 127.0.0.0/8 (127.0.0.1)

Private IPs:

• Class A: 10.0.0.0/8
• Class B: 172.16.0.0/12
• Class C: 192.168.0.0/16

Broadcast: 255.255.255.255

IPv6

IPv6 address is 128-bit (16 bytes), written in hexadecimal.

Format: 2001:0db8:85a3:0000:0000:8a2e:0370:7334

Shortened: 2001:db8:85a3::8a2e:370:7334

Advantages:

• Larger address space
• Better security
• Simplified header

Routing

Routing determines path for packets.

Routing Algorithms

1. Distance Vector

Principle: Each router maintains distance to all destinations.

Algorithm:

• Bellman-Ford
• Periodic updates
• Example: RIP

Problems:

• Count-to-infinity
• Slow convergence

2. Link State

Principle: Each router knows entire network topology.

Algorithm:

• Dijkstra's shortest path
• Flooding of link state
• Example: OSPF

Advantages:

• Faster convergence
• No count-to-infinity

3. Path Vector

Principle: Maintains path information.

Example: BGP

Routing Table

Routing Table contains:

• Destination network
• Next hop
• Interface
• Metric

Fragmentation

Fragmentation divides packets when MTU is smaller.

MTU (Maximum Transmission Unit): Maximum packet size.

Fragmentation Fields:

• Identification
• Flags (MF, DF)
• Fragment offset

Reassembly: Done at destination.

GATE CS Important Points

1. IP Addressing: Classes, subnetting, CIDR
2. Subnet Calculation: Network, broadcast, host addresses
3. Routing Algorithms: Distance vector, link state
4. IPv6: Format, advantages
5. Fragmentation: MTU, fragmentation fields

Practice Tips

1. Subnetting: Practice calculating subnets
2. IP Classes: Know ranges and subnet masks
3. Routing: Understand distance vector vs link state
4. CIDR: Practice CIDR notation
5. Previous Year Questions: Solve GATE network layer questions