Routing primarily occurs at Layer 3, also known as the Network Layer, of the OSI (Open Systems Interconnection) model.
Understanding Layer 3 and Routing
Routers are essential devices that operate at Layer 3. Here's a breakdown of why routing is a Layer 3 function:
- IP Address Inspection: Layer 3 routing relies heavily on the use of IP addresses. Routers examine the IP addresses (and IPX addresses) within the headers of incoming data packets to understand the source and destination.
- Routing Decisions: Based on the destination IP address, routers consult their routing tables to make informed decisions about the best path to forward the packet. Quality of service specifications may also influence these decisions.
- Network Logic: Unlike lower-layer devices such as hubs or switches, Layer 3 devices like routers understand network logic, including subnetting and network segmentation.
- Logical Addressing: Layer 3 operates with logical addresses (IP addresses) and protocols like IP (Internet Protocol).
How Routing Works at Layer 3:
- Packet Arrival: A data packet arrives at a router's interface.
- IP Address Examination: The router looks at the destination IP address within the packet header.
- Routing Table Lookup: The router searches its routing table to find the best next hop for the destination network.
- Forwarding: The packet is forwarded to the next hop router towards the final destination.
- Repeat: This process repeats at each hop until the packet reaches its destination network.
Example:
Imagine a packet from your computer destined for a website. Your local router at your home examines the destination IP address. It consults its routing table and forwards the packet to your ISP's router. This process is repeated as the packet moves through various routers across the internet until it arrives at the server hosting the website.
Why Not Other Layers?
Routing does not take place at lower layers (like Layer 2, the Data Link Layer), which focus on physical addressing (MAC addresses) and moving data within a local network segment. Higher layers (like the Transport or Application layers) concern themselves with end-to-end connections and application-specific details.
