What is a routing table and how does it work?

A router is one of the most essential elements in a computer network. It is responsible for routing data packets to inter-connected networks and end users.

How does a router work?

When a data packet reaches a router, the router reads the address from the packet's header and determines the packet's port to which it needs to be routed.

The router calculates the route of the data packet using a routing table. The routing table helps the router to determine which port the packet will be sent to after the router has processed it. Each router in a network has a unique routing table that is stored in its RAM.

What is a routing table?

A routing table is a critical component of a router's functionality that enables the router to forward the data packets in an intelligent way. It contains all the necessary information about the available paths in an Internet protocol (IP) network.

The routing table is populated with a list of IP addresses that point to different destinations for the packet in transit. When a router receives a packet, it consults the routing table to decide the destination IP address and the best route.

A routing table is structured in the following form:

Attributes of the routing table

A routing table comprises the following attributes:

• Network destination: Each routing table contains an entry for the destination IP address of the packet in transit.

• Netmask: This is a 32-bit network mask, also known as a subnet mask that is used to determine if the packet belongs to the local network or a remote network. Each router uses this network mask to match a destination IP address to the network address.

• Gateway: This entry consists of the IP address of the neighboring device to which the packet would be ultimately forwarded. It is also commonly known as the Next Hop.

• Outgoing interface: This entry contains the router's interface through which the data packet will be forwarded for the next hop

• Metric: This entry contains a value for each available path in the specific network. This allows the router to choose the best path to the specified destination.

Types of routing

The routing table can be populated in two ways which are as follows:

1. Static Routing

2. Dynamic Routing

Static routing

It is a network configuration approach in which the router is manually configured with the available paths and the table elements do not change dynamically. Moreover, no exchange of routing information takes place between routers in static routing.

As shown in the figure above, if the link between routers 1 and 3 is congested or has failed, router 1 would not be able to send packets to router 3 until its routing table is manually updated. After that, the packets will be forwarded to router 3 via router 2.

Dynamic routing

Dynamic routing is an approach where the entries of the routing table are dynamically populated in the router through routing protocols. The routers exchange information with each other about the routing network, thereby, responding to events like network congestion or device failure to provide the best available route for data packets.

If the link fails in dynamic routing, the routing tables of the routers in the network would be updated dynamically to provide the best route. Therefore, the packets would reach router 3 via router 2 if the link between routers 1 and 3 fails.

How does a routing table work?

Now let's take a step-by-step approach to understanding the function of a routing table.

1. When a router receives the packet, it examines the header information and determines the destination IP address of the packet.

2. The router compares the destination address with the entries available in the routing table and finds the most specific match. The router uses the Longest Prefix MatchThe algorithm that compares two addresses to find the entry with the longest prefix that matches to the destination address algorithm for this purpose.

3. After finding a suitable match for the packet in its table, the router retrieves the associated attributes like the next hop and outgoing interface of that entry from the routing table.

4. The router then forwards the packet through the outgoing interface to the designated next router. This process continues until the data packet ultimately reaches its destination.

5. In dynamic routing, the router receives real-time updates from the network and updates its entries to improvise and provide the best possible route to the data packets in transit. This exchange of information takes place in accordance with the routing protocols.

Summary

The routing table is a crucial element in a network that enable routers to make intelligent routing decisions. By storing network destinations and associated attributes, routers can efficiently determine the optimal path for packet forwarding. Whether through static configuration or dynamic updates, routing tables ensure effective communication and efficient data transmission within computer networks.