Lecture 3: Network Layer

Network Layer Basics

• The Network Layer (Layer 3) handles communication between end devices across multiple networks.
• Performs four primary operations: Addressing, Encapsulation, Routing, and De-encapsulation.
• Internet Protocol (IP) is the core protocol (IPv4 and IPv6).
• IP is Connectionless (does not establish a session before sending), operates on a Best Effort basis (unreliable, relies on TCP for error recovery), and is media-independent.

IPv4 vs. IPv6 Headers

• IPv4 addresses are 32-bit. Crucial header fields include Version (0100), Time-to-Live (TTL) (decreases per hop to prevent routing loops), Protocol, Source IP, and Destination IP. IPv4 is facing address depletion, making NAT (Network Address Translation) necessary.
• IPv6 addresses are 128-bit, providing immense address space and eliminating the need for NAT. The IPv6 header is fixed at 40 bytes and removes fields like the Header Checksum and Fragmentation to improve performance. TTL is replaced by Hop Limit, and it includes a Flow Label for traffic handling.

Routing and the Default Gateway

• Host devices have routing tables. If a destination is outside the local network (LAN), the host forwards the packet to the Default Gateway (a router interface on the same local network).
• Routers examine the Destination IP, check their routing tables, and forward the packet.
• Routes can be Directly Connected, Static (configured manually by an admin, good for small networks), or Dynamic (automatically discovered and updated via protocols like OSPF, adapting to topology changes).