In computer networking a routing table is an electronic table (file) or database type object that is stored in a router or a networked computer. The routing table stores the routes (and in some cases, metrics associated with those routes) to a particular network destinations. This information contains the topology of the network immediately around it. It is used to direct forwarding of packets by matching destination addresses in a packet to the network paths in routing table used to reach them. The construction of routing table is the primary goal of routing protocols. This article or section is in need of attention from an expert on the subject. ... A large core router used for major networks. ... The NASA Columbia Supercomputer. ... Diagram of different network topologies. ... Forwarding is the relaying of packets from one network segment to another by nodes in a computer network. ... The introduction to this article provides insufficient context for those unfamiliar with the subject matter. ...

Basics

A routing table utilizes the same idea as using a map in package delivery. Whenever a node needs to send data to another node on a network, it needs to know where to send it. Whenever a device cannot directly connect to the destination node, it must look at a routing table to find another way to send the package. Whenever a node does not know how to send the package it sends an IP packet to a gateway in the LAN. Since this is a complicated task to route the package to the correct destination, a gateway needs to keep track of the way to deliver this. A Routing Table is the way in which this data is stored, like a map. It is a database which keeps track of paths like a map and provides this information to the node requesting the data. ^[1] Node may mean: Node (botany), the place on a plant stem where a leaf is attached Node (physics), a spatial locus along a standing wave where the wave has minimal amplitude Node (networking), a device connected to a network, such as a computer or router Node (computer science), a basic... IP may refer to: IP address (Internet Protocol address), a computer network address Internet Protocol, the computer networking protocol used on the Internet Internet protocol suite or TCP/IP protocol suite, Internet communications protocols Intellectual property: the legal concept that the works created with intellectual effort is a form of... Gateway has several meanings. ... Lan can stand for several things: A local area network Lan (airline) formerly LanChile Lan Peru Län, a kind of administrative division used in Sweden Lan Mandragoran, a fictional character in the Wheel of Time fantasy series by Robert Jordan. ...

Simplest form

In the simplest model, hop-by-hop routing, each routing table lists, for all reachable destinations, the address of the next device along the path to that destination; the next hop. Assuming that the routing tables are consistent, the simple algorithm of relaying packets to their destination's next hop thus suffices to deliver data anywhere in a network. In practice, hop-by-hop routing is being increasingly abandoned in favor of layered architectures such as MPLS, where a single routing table entry can effectively select the next several hops, resulting in reduced table lookups and improved performance. MPLS is a common abbreviation for Multiprotocol Label Switching. ...

Function

During the process of routing, decisions of hosts and routers are aided by a database of routes known as the routing table. The routing table is not exclusive to a router. Depending on the routable protocol, hosts may also have a routing table that may be used to decide the best router for the packet to be forwarded. IP hosts have a routing table. IPX hosts do not have a routing table.

The types of entries in a routing table:

Network Route: A route (path) to a specific Network ID in the internetwork.

Host Route : A route to a specific internetwork address (Network ID and Host ID). Host routes allow intelligent routing decisions to be made for each network address. Host routes are used to create custom routes to control or optimize specific types of network traffic.

Default Route: A route that is used when no other routes for the destination are found in the routing table. If a router or end system (such as a PC running Windows), cannot find a route for a destination, the default route is used.

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Difficulties with Routing Tables

The need to record routes to large numbers of devices using limited storage space represents a major challenge in routing table construction. A fundamental assumption of many routing schemes is that similar addresses are located near each other in the network, allowing groups of destination addresses to be matched by single routing table entries. The exact nature of how this grouping is done has changed over time and still represents an active area of networking research. In the Internet, the currently dominant address grouping technology is a bitwise prefix matching scheme called Classless Inter-Domain Routing. It has been suggested that Routing protocol be merged into this article or section. ... It has been suggested that this article or section be merged with Provider-based addressing. ...

Since in a network each node presumably possesses a valid routing table, routing tables must be consistent among the various nodes or routing loops can develop. This is particularly problematic in the hop-by-hop routing model in which the net effect of inconsistent tables in several different routers could be to forward packets in an endless loop. Routing loops have historically plagued routing, and their avoidance is a major design goal of routing protocols.

Contents of Routing Tables

The routing table consists of at least three information fields:-

1. the network id: i.e. the destination network id
2. cost: i.e. the cost of the path through which the packect is to be sent .
3. next hop:next hop is the address of the next station to which the packet is to be sent on the way to destination .

Depending on the application and implementation, it can also contain additional values that refine path selection:

1. quality of service associated with the route
2. links to filtering criteria/access lists associated with the route

Routing tables also are a key aspect of certain security operations, such as unicast reverse path forwarding (uRPF) ^[2]. In this technique, which has several variants, the router also looks up, in the routing table, the source address of the packet. If there exists no route back to the source address, the packet is assumed to be malformed or an attack, and is dropped.

See also

• Forwarding table
• packet forwarding

Forwarding is the relaying of packets from one network segment to another by nodes in a computer network. ...

References

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External links

• Good explanation of the Network destination, Netmask, Gateway, Interface, Metric, etc.