A routing table is utilized by TCP/IP network routers to evaluate the destinations of the data packets to be forwarded. It is a small in-memory database controlled by the router's built-in hardware and software that contains the necessary data to forward a packet to its destination, following an optimum path.
Each packet consists 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.
A routing table is used by all IP-enabled devices, like routers and switches. It comprises 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 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 home routers install their routing table automatically when connected to the ISP (Internet Service Provider). They give one entry for individual ISPs and DNS servers, and one for 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 tables automatically using 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 business routers permit network administrators to manually update or manipulate routing tables, which is not possible in residential network routers. 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 each node possesses a valid routing table, the tables must be consistent among different nodes, else loops may develop. This can be problematic in the hop-by-hop model, in which the net effect of inconsistent tables in various routers could be to transmit packets in an endless loop. These have plagued routing since a long time, and are a major concern.
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 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 dropped.