Lesson 16: OSI Layer 3: The Network Layer (Routing)

Layer 3 is arguably the most crucial layer for inter-network communication. It is responsible for logical addressing and determining the best path (routing) across multiple networks.

Core Function: Logical Addressing and Path Determination

Data units at this layer are called Packets.

Key Functions:

1. Logical Addressing: Providing the primary addressing scheme for the global internet: the IP Address (Internet Protocol address).
2. Routing: Using routing tables to determine the optimal path for a packet to travel from the source network to the destination network, often passing through many routers.

Key Protocol and Devices:

• Primary Protocol: Internet Protocol (IP) (IPv4 and IPv6).
• Key Device: Router (Routers work exclusively at Layer 3).

How Routing Works:

If the destination IP address is on the same local network, the packet is sent directly. If the destination IP address is on a remote network, the router looks up the destination network ID in its routing table and sends the packet to the next hop (the next router in the path).

Analogy: The Network Layer is the GPS system. It looks at the destination country and city (IP Address) and plots the best route across the highway system (routers) to get the package (packet) to the correct location.