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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
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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
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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.
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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
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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.
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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.
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indicates the associated cost of using the indicated route. This is useful for determining the efficiency of a certain route from two points in a network. In this example, it is more efficient to communicate with the computer itself through the use of address
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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
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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
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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.
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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
196:: 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
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function of the forwarding table. This separation of control and forwarding provides uninterrupted high-performance forwarding.
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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.
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Depending on the application and implementation, it can also contain additional values that refine path selection:
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are entries that are fixed, rather than resulting from routing protocols and network topology discovery procedures.
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Shown below is an example of what the table above could look like on a computer connected to the internet via a
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indicates what locally available interface is responsible for reaching the gateway. In this example, gateway
144:(uRPF). In this technique, which has several variants, the router also looks up, in the routing table, the
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as preferred routes for packet forwarding. It is often in a compressed or pre-compiled format that is
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in modern router architectures; instead, they are used to generate the information for a simpler
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interface type and number, are entered into the routing table as a directly connected network.
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70:(distances) associated with those routes. The routing table contains information about the
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117:. Assuming that the routing tables are consistent, the simple algorithm of relaying
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associated with the route. For example, the U flag indicates that an IP route is up.
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223:: Such as eth0 for the first Ethernet card, eth1 for the second Ethernet card, etc.
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390:(the internet router) can be reached through the local network card with address
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Routing tables are also a key aspect of certain security operations, such as
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that lists the routes to particular network destinations, and in some cases,
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Static routes are routes that a network administrator manually configured.
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Data table stored in a router that lists the routes to network destinations
431:. This forwarding table contains only the routes which are chosen by the
379:, i.e. it points to the gateway through which the network can be reached.
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needs to send data to another node on a network, it must first know
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The routing table consists of at least three information fields:
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Forwarding and
Control Element Separation (ForCES) Framework
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The construction of routing tables is the primary goal of
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A routing table is analogous to a distribution map in
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can also be used to help control routing table size.
423:Routing tables are generally not used directly for
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356:as mentioned earlier. For example, destination
375:column contains the same information as the
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414:(the IP address of the local network card).
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516:Ingress Filtering for Multihomed Networks
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513:F. Baker & P. Savola (March 2004).
450:function of the routing table from the
446:This router architecture separates the
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180:: The destination subnet and netmask
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235:Example routing table contents
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142:unicast reverse path forwarding
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490:Requirements for IPv4 Routers]
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159:Classless Inter-Domain Routing
1:
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30:Route table showing internet
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410:) than it would be through
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10:
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21:Router table (woodworking)
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439:for hardware storage and
217:associated with the route
48:routing information base
19:Not to be confused with
469:Internet protocol suite
74:immediately around it.
72:topology of the network
553:, RFC3746, April 2004.
488:F. Baker (June 1995).
352:together describe the
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583:Internet architecture
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240:Network destination
215:Access-control lists
346:Network destination
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40:computer networking
364:can be written as
354:Network identifier
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211:filtering criteria
205:quality of service
178:network identifier
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433:routing algorithm
425:packet forwarding
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79:routing protocols
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526:10.17487/RFC3704
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452:forwarding plane
429:forwarding table
419:Forwarding table
328:255.255.255.255
311:255.255.255.255
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95:package delivery
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123:Internet layer
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448:control plane
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397:Finally, the
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97:. Whenever a
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360:and netmask
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344:The columns
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152:Difficulties
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125:and the OSI
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64:network host
58:stored in a
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388:192.168.0.1
358:192.168.0.0
229:home router
577:Categories
567:IP Routing
549:, L. Yang
475:References
317:127.0.0.1
314:127.0.0.1
283:127.0.0.1
280:127.0.0.1
277:255.0.0.0
274:127.0.0.0
249:Interface
56:data table
437:optimized
408:localhost
404:127.0.0.1
384:Interface
221:interface
458:See also
406:(called
377:Next hop
260:0.0.0.0
257:0.0.0.0
246:Gateway
243:Netmask
194:next hop
169:Contents
161:(CIDR).
89:Overview
54:), is a
588:Routing
373:Gateway
350:Netmask
252:Metric
119:packets
68:metrics
34:routes
551:et al.
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441:lookup
399:Metric
186:: The
184:metric
60:router
103:where
62:or a
46:, or
534:3704
499:1812
382:The
371:The
348:and
99:node
42:, a
531:RFC
521:doi
494:RFC
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269:10
115:hop
52:RIB
38:In
32:BGP
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231::
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50:(
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