Network Routing Table - What is a Routing Table

A routing table is utilized by TCP/IP network routers, to evaluate the destinations of the data packets it is supposed to forward. It is a small in-memory database controlled by the router's built-in hardware and software. A routing table is a set of rules which helps in determining where the data packets passing over an internet protocol network will be directed. The routing table contains the necessary data to forward a packet to its destination by following an optimum path. Each packet comprises of information about its origin and destination. When the packet is received, a network device analyzes the packet and matches it to the routing table entry to provide the best match for its destination. It provides the device with protocols for transmitting the packet to the next hop or IP address, on its route across the network. The routing table is used by all IP enabled devices like routers and switches. A routing table comprises of the following information:

• Destination: It provides the IP address of the final destination.
• Next Hop: It provides the IP address to which the packet is transmitted.
• Interface: It is used while transmitting the packet to the next hop or final destination.
• Metric: Assigns a rank to each available route as per their cost so that the most cost-effective path can be chosen.
• Routes: Comprises of directly-attached subnets, indirect subnets which are not connected to the device but can be accessed through one or more hops and default routes that are used for certain types of traffic or when information is missing.

Dynamic and Static Routing

Dynamic routing is a process in which the home routers install their routing table automatically when connected to the ISP (Internet Service Provider). They give one entry for individual ISPs, DNS servers and one for the routing amongst the home computers. They can also provide extra routes for other cases like multicast and broadcast routes. In dynamic routing, the devices build and assert their routing tables automatically by using routing protocols to exchange data regarding the surrounding network topology. These tables allow the devices to listen to the network and respond to events like device failures and network congestion.

Static routing is a process in which the business routers permit the network administrators to manually update or manipulate the routing tables, which is not possible in residential network routers as they prevent you from overriding or changing the routing table. Tables for static network devices remain constant, unless a network administrator manually changes them. It helps in optimizing the network performance and reliability.

Difficulties with Routing Tables

The necessity to record routes to many devices using restricted storage space, is a big challenge in routing table construction. Presently, the dominant address aggregation technology consists of a bitwise prefix matching scheme, commonly known as Class Inter-Domain Routing (CIDR). Since in a network, each node possesses a valid routing table, the tables must be consistent among the different nodes else routing loops may develop. This can be problematic in the hop-by-hop routing model, in which the net effect of inconsistent tables in various routers could be to transmit packets in an endless loop. These loops have plagued routing since a long time, and their avoidance is a major design concern of routing protocols.

Routing tables are a key element of many security operations, like unicast reverse path forwarding (uRPF). This method has several versions in which the router searches the routing table for the source address of the packet. If there exists no route back to the source address, the packet is considered distorted or involved in a network attack and is finally dropped.