Routing

When you send a message to someone on the same local network, the data may reach them directly. But what happens when you want to access a server located on another network or another side of the world?

The data needs to find a path through multiple networks. This process is called Routing.

Routing is the process of determining and using a path for forwarding packets from a source network toward a destination network.

Routers are the main devices responsible for routing.

Computer
    ↓
 Router A
    ↓
 Router B
    ↓
 Router C
    ↓
Web Server

Each router examines the destination IP address and decides where the packet should go next.

Why Do We Need Routing?

The Internet is not one giant network. It is made up of millions of interconnected networks.

If your computer wants to communicate with a server on another network, it needs a way to reach that network.

Routing helps:

• Find paths between networks

• Forward packets toward their destinations

• Connect different networks

• Choose appropriate next hops

• Keep large networks organized

Without routing, devices would have no practical way to communicate across different IP networks.

How Does Routing Work?

Suppose your computer wants to communicate with a server:

Your PC
192.168.1.10
     |
     ↓
Router
     |
     ↓
Internet
     |
     ↓
Server

Your computer checks whether the destination is on its local network.

If the destination is on another network, the packet is sent to the default gateway, usually a router.

The router then checks its routing table to determine where to forward the packet.

Packet arrives
      ↓
Check destination IP
      ↓
Look at routing table
      ↓
Choose next hop/interface
      ↓
Forward packet

This process is repeated by routers along the path until the packet reaches its destination network.

What Is a Routing Table?

A routing table is a collection of information that a router uses to decide where to forward packets.

A simplified routing table might look like:

When a packet arrives, the router compares the destination IP address with its routing entries and selects the most specific matching route.

What Is a Next Hop?

A next hop is the next router or destination to which a packet should be forwarded.

For example:

Router A → Router B → Router C → Destination
             ↑
          Next Hop

From Router A's perspective, Router B is the next hop.

After Router B receives the packet, it makes its own routing decision.

Types of Routing

Routing is commonly classified into:

1. Static Routing

2. Dynamic Routing

1. Static Routing

In static routing, routes are manually configured by a network administrator.

For example:

Network: 10.0.0.0/8
Next Hop: 192.168.1.1

The administrator explicitly tells the router where to send traffic for that network.

Advantages

• Simple for small networks

• No routing-protocol overhead

• Gives administrators direct control

Disadvantages

• Manual configuration

• Difficult to maintain in large networks

• Does not automatically adapt to network failures or topology changes

Static routing is useful when the network is small or the routing requirements are simple and predictable.

2. Dynamic Routing

In dynamic routing, routers use routing protocols to exchange information and learn routes automatically.

Some well-known routing protocols include:

• RIP

• OSPF

• BGP

• EIGRP

For example:

Router A ←→ Router B
    ↕          ↕
Router C ←→ Router D

Routers exchange routing information and can update their routing tables when the network changes.

Advantages

• Automatically learns routes

• Adapts to network changes

• Suitable for larger networks

Disadvantages

• More complex

• Uses network and processing resources

• Requires understanding of routing protocols

Routing Protocols

Different routing protocols use different methods to determine the best paths.

RIP

RIP (Routing Information Protocol) uses hop count as its primary metric.

A route with fewer hops is generally preferred.

RIP is simple but has limitations and is not suitable for many modern large networks.

OSPF

OSPF (Open Shortest Path First) is a link-state routing protocol commonly used within organizations and enterprise networks.

It uses a cost metric and builds a view of the network topology to calculate routes.

BGP

BGP (Border Gateway Protocol) is used to exchange routing information between autonomous systems and is fundamental to Internet routing.

It uses policies and path attributes to select routes rather than simply choosing the physically shortest path.

Routing vs Forwarding

These two terms are closely related but are not exactly the same.

Routing

Routing is the process of determining paths and maintaining routing information.

Forwarding

Forwarding is the actual process of moving a packet from an incoming interface to the appropriate outgoing interface.

Think of it like this:

Routing → Decides the path

Forwarding → Moves the packet

Routing vs Switching

Routing and switching are also commonly confused.

For example:

PC ── Switch ── Router ── Internet
       ↑           ↑
   Switching    Routing

The switch handles local Ethernet forwarding, while the router handles forwarding between IP networks.

Default Route

Sometimes a router does not have a specific route for a destination.

In that situation, it can use a default route if one is configured.

For IPv4, the default route is commonly written as:

0.0.0.0/0

For IPv6, it is:

::/0

A default route essentially means:

"If no more specific route matches, send the packet this way."

For example, a home router typically has a default route toward the ISP.

Simple Real-Life Example

Suppose you open a website hosted on another network.

The process can be simplified as:

Your Computer
      ↓
Default Gateway
      ↓
Router 1
      ↓
Router 2
      ↓
Router 3
      ↓
Web Server

At each routing step, the router examines the destination IP address and decides where to forward the packet next.

The exact path may change depending on routing information and network conditions.

Conclusion

Routing is the process of determining paths and forwarding packets between different networks.

Routers use routing tables to make forwarding decisions, and routes can be configured manually using static routing or learned automatically using dynamic routing protocols.

The easiest way to remember the concept is:

Routing → Decide where the packet should go

Forwarding → Send the packet to the next destination

Once you understand routing, concepts such as routing tables, default gateways, static routes, OSPF, BGP, and autonomous systems become much easier to understand.