Network Layer in OSI Model

Network Layer is the third layer from the bottom (Layer 3) and the fifth layer from the top in the OSI (Open Systems Interconnection) Model. It is responsible for ensuring end-to-end packet delivery across multiple interconnected networks.

Note: Unlike the Data Link Layer, which focuses only on node-to-node delivery within a single network segment, the Network Layer ensures that data travels from the source host to the destination host, even if they are located on different networks.

Key Responsibilities of the Network Layer

• Logical Addressing: Assigns unique IP addresses to devices, ensuring accurate identification and communication across networks.
• Packetization: Encapsulates transport layer segments into packets for efficient transmission.
• Host-to-Host Delivery: Ensures reliable delivery of packets from the sender to the intended receiver across diverse networks.
• Forwarding: Moves packets from the input interface of a router to the appropriate output interface based on the destination IP.
• Routing: Determines the optimal path for packets to travel across multiple networks using routing algorithms and protocols.
• Fragmentation and Reassembly: Splits large packets into smaller fragments to match the maximum transmission unit (MTU) of a network, and reassembles them at the destination.
• Subnetting: Divides larger networks into smaller subnetworks for efficient addressing and traffic management.
• Network Address Translation (NAT): Maps private IPs to public IPs for internet communication, conserving address space and adding security.

Read more about Functions of Network Layer.

How the Network Layer Works

• Each device is assigned a unique logical address (IP address).
• Data from the transport layer is encapsulated into packets, with source and destination IPs attached.
• Routers analyze the destination address and determine the best available path.
• Packets traverse the network hop-by-hop, moving across routers until reaching the destination.
• If the packet size exceeds the MTU, it is fragmented into smaller units.
• At the destination, the fragments are reassembled into the original data.
• If errors occur (e.g., unreachable destination), protocols like ICMP send error messages back to the source.

Protocols Operating at the Network Layer

• IP (Internet Protocol – IPv4/IPv6)
• ICMP (Internet Control Message Protocol)
• ARP (Address Resolution Protocol)
• RARP (Reverse Address Resolution Protocol)
• NAT (Network Address Translation)
• IPSec (Internet Protocol Security)
• MPLS (Multiprotocol Label Switching)

Routing Protocols

• RIP (Routing Information Protocol)
• OSPF (Open Shortest Path First)
• BGP (Border Gateway Protocol)

Advantages of the Network Layer

• Enables end-to-end communication across multiple networks.
• Supports scalability by allowing subnetting and hierarchical addressing.
• Efficiently routes packets using shortest-path and dynamic routing algorithms.
• Provides inter-networking by connecting heterogeneous networks.

Limitations of the Network Layer

• No flow control mechanism; congestion may occur if too many datagrams are in transit.
• Limited error control; mainly relies on upper layers for reliability.
• Routers may drop packets under heavy load, leading to possible data loss.
• Fragmentation increases processing overhead and may affect performance.

Difference Between Routing and Flooding

For practice, solve quiz on Network Layer.