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). 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. Difficulties The need to record routes to large numbers of devices using limited storage space represents a major challenge in routing table construction. In the Internet, the currently dominant address aggregation technology is a bitwise prefix matching scheme called Classless Inter-Domain Routing (CIDR). Supernetworks can also be used to help control routing table size. Contents The routing table consists of at least three information fields: network identifier: The destination subnet and netmask metric: The routing metric of the path through which the packet is to be sent. The route will go in the direction of the gateway with the lowest metric. next hop: The next hop, or gateway, is the address of the next station to which the packet is to be sent on the way to its final destination Depending on the application and implementation, it can also contain additional values that refine path selection: quality of service associated with the route. For example, the U flag indicates that an IP route is up. filtering criteria: Access-control lists associated with the route interface: Such as eth0 for the first Ethernet card, eth1 for the second Ethernet card, etc. Shown below is an example of what the table above could look like on a computer connected to the internet via a home router: The columns Network destination and Netmask