Editing Routing table (section)

==Overview==
A routing table is analogous to a distribution map in [[package delivery]]. Whenever a [[Node (networking)|node]] needs to send data to another node on a network, it must first know ''where'' to send it. If the node cannot directly connect to the destination node, it has to send it via other nodes along a route to the destination node. Each node needs to keep track of which way to deliver various packages of data, and for this it uses a routing table. A routing table is a database that keeps track of paths, like a map, and uses these to determine which way to forward traffic. A routing table is a data file in RAM that is used to store route information about directly connected and remote networks. Nodes can also share the contents of their routing table with other nodes.

The primary function of a router is to forward a packet toward its destination network, which is the destination IP address of the packet. To do this, a router needs to search the routing information stored in its routing table. The routing table contains network/next hop associations. These associations tell a router that a particular destination can be optimally reached by sending the packet to a specific router that represents the next hop on the way to the final destination. The next hop association can also be the outgoing or exit interface to the final destination.

With 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 (networking)|hop]].  Assuming that the routing tables are consistent, the simple algorithm of relaying [[Packet (information technology)|packets]] to their destination's next hop thus suffices to deliver data anywhere in a network.  Hop-by-hop is the fundamental characteristic of the IP [[Internet layer]]<ref>{{cite IETF |title=Requirements for IPv4 Routers] |author=F. Baker |RFC=1812 |date=June 1995}}</ref> and the OSI [[Network Layer]].

When a router interface is configured with an IP address and subnet mask, the interface becomes a host on that attached network. A directly connected network is a network that is directly attached to one of the router interfaces. The network address and subnet mask of the interface, along with the interface type and number, are entered into the routing table as a directly connected network.

A remote network is a network that can only be reached by sending the packet to another router. Routing table entries to remote networks may be either dynamic or static. Dynamic routes are routes to remote networks that were learned automatically by the router through a dynamic routing protocol. Static routes are routes that a network administrator manually configured.

Routing tables are also a key aspect of certain security operations, such as [[unicast reverse path forwarding]] (uRPF).<ref>{{cite IETF |rfc=3704 |title=Ingress Filtering for Multihomed Networks |author=F. Baker & P. Savola |date=March 2004}}</ref> 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 involved in a network attack and is dropped.