What is a Routing Table?

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? It comprises the routing entries, also known as routes, which the router acquires through different means. The router can 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 set of rules, often viewed in table format, that’s used to determine where data packets traveling over an Internet Protocol (IP) network will be directed. This table is usually stored inside the Random Access Memory of forwarding devices, such as routers and network switches.

In computer networking, each routing table is unique and acts as an address map for networks. It stores the source and destination IP addresses of the routing devices in the form of prefixes along with the default gateway addresses and corresponding routing information.

Routing tables are typically updated dynamically through network routing protocols. But sometimes network administrators might add static entries manually.

How does it work?

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.

Benefits of Routing Tables

• Efficient Data Routing: It allows packets to be routed across the network in the most efficient way possible.
• Scalability: Supports large and complex networks by managing multiple paths and dynamic routing protocols.
• Flexibility: Can be configured manually or updated automatically to adapt to changes in the network.

Entries of an IP Routing Table

It stores the information required to forward the data packet along the best path toward its destination. Each data packet has a piece of information about its origin and its destination each entry of the routing table contains entries:

• Network ID: The network ID or destination that corresponds to the route.
• Subnet Mask: The subnet mask is the mask required to match the destination IP address with the network ID address.
• Next Hop: It is an IP address to which the data packet is to be forwarded and it specifies the next router which is closest/most optimal in its routing path.
• Outgoing Interface: An outgoing interface is an interface through which the data packet should go out to reach the destination network.
• Metric: The general use of the metric is to specify the count of the minimum number of hops (routers crossed) to the network ID.

The following types of routes can be stored in the routing table entries

• Directly Attached Network IDs
• Remote Network IDs
• Host Routes
• Default Route
• Destination

Considerations and Challenges

• Complexity in Large Networks: Managing routing tables can become complex as the network grows.
• Security Risks: Incorrect configurations or compromised routing protocols can lead to data being sent through insecure or inefficient paths.
• Performance Issues: Large routing tables can consume considerable resources, impacting the performance of network devices.

Conclusion

This article has provided an overview of understanding the entries in routing tables. First, we defined routers, and then routing tables. We discussed how routing tables are generated, and where they’re stored. We also explained the components of routing table entries and gave examples of how to decipher these entries. Finally, we looked at the types of routes that are usually recorded in routing tables.