Network Routing

What is Network Routing?

Network routing is the process of selecting paths in a network along which to send network traffic. Routers are the network devices that perform this function. They use routing tables to determine the best path to forward data packets from their source to their destination.

Imagine a postal service. When you send a letter, the post office doesn't just send it directly to the recipient. It goes through various sorting centers and transport routes. Network routing is similar, but for digital data. Routers are the "sorting centers" and "transport hubs" that direct your data packets efficiently across the internet or a private network.

Key Concepts

Routers

Routers are specialized network devices that connect two or more networks. Their primary role is to examine the destination IP address of a packet and use their routing table to decide where to forward it next. They operate at Layer 3 (the Network Layer) of the OSI model.

Routing Tables

A routing table is a data table stored in a router that lists the routes to particular network destinations. It contains information like:

• Destination Network: The network address to which a route is defined.
• Next Hop: The IP address of the next router in the path.
• Interface: The outgoing network interface on the current router to use for this route.
• Metric: A value indicating the "cost" or "preference" of a route (e.g., hop count, bandwidth, delay). Lower metrics are generally preferred.

Example of a simplified routing table entry:

Destination Network | Next Hop        | Interface | Metric
------------------|-----------------|-----------|-------
192.168.1.0/24    | 10.0.0.1        | Eth0      | 2
172.16.0.0/16     | 10.0.0.2        | Eth1      | 5
Default Route     | 192.168.0.1     | Eth0      | 1
            

IP Addresses

Routing relies heavily on IP addresses. The destination IP address in a packet's header is the key piece of information a router uses to make forwarding decisions.

Subnetting

Subnetting is a technique used to divide a larger IP network into smaller subnets. This improves network organization, security, and efficiency, and is fundamental to effective routing.

How Routing Works

When a router receives a data packet:

1. It examines the destination IP address in the packet header.
2. It consults its routing table to find the best match for the destination IP address.
3. If a direct match is found, the packet is forwarded to the specified next hop or interface.
4. If no direct match is found, the router typically uses a "default route" (often represented as 0.0.0.0/0) to forward the packet to a designated gateway router.
5. The packet is then sent out on the appropriate outgoing interface towards its destination.

Routers do not inspect the contents of the data packets; they only look at the Layer 3 (IP) header information.

Types of Routing

Static Routing

In static routing, network administrators manually configure routes in the routing table. These routes remain fixed unless manually changed. Static routing is simple, secure, and efficient for small, stable networks but is not scalable for larger or dynamic environments.

Dynamic Routing

Dynamic routing uses routing protocols to automatically learn and update routing information. Routers running the same routing protocol exchange information with each other to build and maintain their routing tables. This is essential for larger and changing networks.

Common Dynamic Routing Protocols:

• RIP (Routing Information Protocol): A distance-vector protocol, simple but less efficient for large networks.
• OSPF (Open Shortest Path First): A link-state protocol, more complex but highly scalable and efficient.
• BGP (Border Gateway Protocol): The routing protocol of the internet, used for routing between Autonomous Systems (AS).

The choice of routing protocol depends on the network's size, complexity, and performance requirements.

Routing vs. Switching

It's important to distinguish routing from switching:

Feature	Routing	Switching
OSI Layer	Layer 3 (Network Layer)	Layer 2 (Data Link Layer)
Addressing Used	IP Addresses	MAC Addresses
Device	Router	Switch
Function	Connects different networks, determines paths between networks.	Connects devices within the same network, forwards frames based on MAC addresses.
Broadcast Domain	Segments broadcast domains.	Operates within a broadcast domain.

Routers make decisions based on IP addresses to move traffic between networks, while switches make decisions based on MAC addresses to move traffic within a single network.