What is a Routing Table in Computer Networks?

Introduction

Routing is a fundamental concept in the field of data communication networks. Routing allows the transmission of data from a specific source address to a designated destination address across a network that is interconnected. Routing takes place at the network layer, specifically Layer 3, within the framework of the Open Systems Interconnection (OSI) reference model. But how does a router know where to send a packet? The answer is: by using a routing table. But what is it?

The routing table comprises the routing entries, also known as routes, which the router acquires through different means. The router has the capability to acquire routing entries through either static or dynamic means and subsequently manage its own routing table.

In this blog, we will focus on the basic functioning of the routing table in networking, its components, and its working.

What is a Routing Table?

A routing table is a data structure that allows the storage and access of necessary data regarding routes leading to designated IP destinations. The established pathways are afterwards utilized to transmit packets across network links in order to reach the right destination. Routers and switches utilize route tables in order to determine the best route for forwarding data packets within a network.

In simple terms, a routing table in networking is like a map that helps routers and switches make decisions about where to send data packets. Each entry in the table contains information about the network addresses and the corresponding next-hop or outgoing interface for forwarding the packets.

These tables are dynamic and can be updated based on changes in the network topology or network conditions. This allows network devices to adapt to changes and find new paths if the previous routes become unavailable or congested.

Why are Routing Tables Important?

• Routing tables make sure that data packets reach their destination by taking the shortest, fastest, and most effective path.
• They play a crucial role in reducing communication latency by minimizing delays and optimizing routes.
• They are also helpful in reducing the burden on network administrators as they can automatically handle routing complexities.
• It also ensures that the network infrastructure is scalable, efficient, and capable of adapting to changes.

Routing Table Entries

A route to a certain network is often represented by an entry in a routing table. It consists of multiple entries, each of which contains the following information:

1. Destination Address: The destination address involves the Internet Protocol (IP) address of the network that is being targeted, i.e., the destination network.
2. Subnet mask or Netmask: A subnet mask or netmask is a 32-bit address that indicates which bits of an IP address belongs to the network and which bits belong to the host. A routing table uses the subnet mask to match the destination IP address with the network ID address.
3. Gateway/Next hop: The IP address or interface of the next router or device along the path to the destination network.
4. Interface: The name or number of the local interface that is used to send packets to the next hop.
5. Metric: A numerical value indicating the cost or preference of a certain route. Lower values indicate more preferred routes.

Routing Table Routes

Mainly there are three types of routes in the routing table in networking, these are:

1. Static Routing
2. Dynamic Routing
3. Default Routing

Let’s understand the three routes mentioned above in detail.

Static Routing

In the case of static routing, all the routes are added manually and won’t change unless one intends to modify them.

Advantages of Static Routing:

• There is no need for a more expensive router.
• It ensures security by preventing anybody except the administrator from routing traffic to certain networks.
• There is zero use of bandwidth between the routers.

Disadvantages of Static Routing:

• When managing a big network, it becomes challenging for the administrator to manually enter each route into each router’s routing table.
• Adding each route manually requires the administrator to have an excellent understanding of topology.

Dynamic Routing

In the case of dynamic routing, routes are learned or updated by routing protocols. Routing protocols exchange routing information with other routers and automatically adjust the routing table based on network changes.

Advantages:

• It’s simple to set up dynamic routing.
• Choosing the optimal path to a distant network and locating a remote network are both possible with the help of dynamic routing.

Disadvantages:

• With dynamic routing, you’ll need more bandwidth for communicating with your neighbors.
• When compared to static routing, dynamic routing presents a greater security risk.

Default Routing

Default routing is a method of configuring a router to forward packets to a single destination when there is no specific route for the destination in the routing table.

Advantages:

• With the help of default routing, the routing table stays compact and small.

Disadvantages:

• Default routing might be difficult to set up in a complicated computer network.
• It’s not a reliable routing method since packets won’t be transferred if the next router breaks.

How Does Routing Table Works?

The routing table in networking contains data that is utilized to determine the optimal path for a packet to traverse in order to reach its final destination. Every packet contains both the source and destination IP addresses. Upon receiving a packet, the network device or router examines the IP packet’s contents and compares it with the entries listed in the routing table. The selection of the most suitable entry for the destination is employed to determine the subsequent routing of the packet.

Let’s further understand with the help of an example.

The picture above depicts a network consisting of two PCs and a router. Host A is focused on establishing communication with Host B. Host A must utilize the router as the default gateway since it and the receiving host are on separate subnets. The router gets the packet, looks at the IP address, and analyzes its routing database to determine which interface to send it out. The router’s routing table can be shown with the help of the “show ip route” command.

C 1.0.0.0/8 is directly connected, GigabitRthernet0/1

The above statement indicates that every packet intended for the 10.0.0.0/8 network will be transmitted through the Fa0/1 interface.

And three types of routes that we discussed above are used to populate the table. These are static, dynamic, and default routing.

It’s important to remember that the routing tables don’t just apply to Cisco devices. The “route print” command can also show you the routing table in your Windows. Below we have provided the image.

We hope this explanation was helpful to understand how it works.

Common Routing Protocols

Here are some commonly used routing protocols:

Routing Information Protocol

RIP is one of the oldest routing protocols that is still used in networking. It determines routes based on hop counts, with a maximum of 15 hops. So, it is more suitable for smaller networks.

Open Shortest Path First

OSPF Protocol is a link-state routing protocol that is based on Dijkstra’s algorithm. It calculates the shortest path for data packets and is suitable for large-scale networks.

Border Gateway Protocol

BGP is a routing protocol that powers the internet. It is used to exchange routing information between Autonomous Systems (AS).

Frequently Asked Questions

Q1. What is the routing table?

A routing table is a data structure that stores information about the possible destinations for network packets. It contains a list of routes, each with an associated address or prefix, and a next hop or gateway that indicates where to send the packets that match the route.

Q2. What is the use of router table?

The purpose of a routing table in networking is to help routers and other network devices to forward packets efficiently and correctly to their intended destinations.

Q3. What are the 3 types of routes in routing table?

A routing table in networking has three types of routes: static, dynamic, and default.

Q4. What is routing and how it works?

Routing is the process of directing network traffic from its source to its destination by determining the optimal path based on routing tables, allowing efficient communication between devices in a network. It works by examining the destination IP address of packets and using the table to determine the next-hop or outgoing interface for forwarding the packets towards their intended destination.

Conclusion

A routing table in networking is a vital component of a router that enables it to perform routing efficiently and effectively. It stores information about different routes to network destinations and helps a router determine the best path for a packet based on various criteria. Understanding the significance of routing tables helps us appreciate the underlying mechanisms that enable the vast interconnectedness of modern networks.