IP routing is the application of traffic

routing Routing is the process of selecting a path for traffic in a Network theory, network or between or across multiple networks. Broadly, routing is performed in many types of networks, including circuit-switched networks, such as the public switched ...

methodologies to

IP network The Internet protocol suite, commonly known as TCP/IP, is a framework for organizing the communication protocols used in the Internet and similar computer networks according to functional criteria. The foundational protocols in the suite are ...

s. This involves technologies, protocols, structure, administrations, and policies of the worldwide

Internet The Internet (or internet) is the Global network, global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a internetworking, network of networks ...

infrastructure. In each IP

network node In networking, a node (, ‘knot’) is either a redistribution point or a communication endpoint within telecommunication networks. A physical network node is an electronic device that is attached to a network, and is capable of creating, rec ...

, IP routing involves the determination of a suitable path for a

network packet In telecommunications and computer networking, a network packet is a formatted unit of Data (computing), data carried by a packet-switched network. A packet consists of control information and user data; the latter is also known as the ''Payload ...

from a source to its destination. The process uses rules, obtained from either static configuration or dynamically with

routing protocol A routing protocol specifies how routers communicate with each other to distribute information that enables them to select paths between nodes on a computer network. Routers perform the traffic directing functions on the Internet; data packet ...

s, to select specific

packet forwarding Packet forwarding is the relaying of packets from one network segment to another by nodes in a computer network. Models The simplest forwarding model unicastinginvolves a packet being relayed from link to link along a chain leading from th ...

methods to direct traffic to the next available intermediate network node one ''hop'' closer to the desired final destination. The total path potentially spans multiple

computer network A computer network is a collection of communicating computers and other devices, such as printers and smart phones. In order to communicate, the computers and devices must be connected by wired media like copper cables, optical fibers, or b ...

s. Networks are separated from each other by specialized hosts, called gateways or routers with specialized software support optimized for routing. IP forwarding algorithms in most routing software determine a route through a

shortest path In graph theory, the shortest path problem is the problem of finding a path between two vertices (or nodes) in a graph such that the sum of the weights of its constituent edges is minimized. The problem of finding the shortest path between two ...

algorithm. In routers, packets arriving at an interface are examined for source and destination addressing and queued to the appropriate outgoing interface according to their destination address and a set of rules and performance metrics. Rules are encoded in a

routing table In computer networking, a routing table, or routing information base (RIB), is a data table stored in a router or a network host that lists the routes to particular network destinations, and in some cases, metrics (distances) associated wi ...

that contains entries for all interfaces and their connected networks. If no rule satisfies the requirements for a network packet, it is forwarded to a

default route In computer networking, the default route is a configuration of the Internet Protocol (IP) that establishes a forwarding rule for packets when no specific address of a next-hop host is available from the routing table or other routing mechanism ...

. Routing tables are maintained either manually by a network administrator, or updated dynamically by a

. A routing protocol specifies how routers communicate and share information about the topology of the network, and the capabilities of each routing node. Different protocols are often used for different topologies or different application areas. For example, the

Open Shortest Path First Open Shortest Path First (OSPF) is a routing protocol for Internet Protocol (IP) networks. It uses a link state routing (LSR) algorithm and falls into the group of interior gateway protocols (IGPs), operating within a single Autonomous syste ...

(OSPF) protocol is generally used within an enterprise and the

Border Gateway Protocol Border Gateway Protocol (BGP) is a standardized exterior gateway protocol designed to exchange routing and reachability information among autonomous systems (AS) on the Internet. BGP is classified as a path-vector routing protocol, and it ...

(BGP) is used on a global scale. BGP is the

de facto standard A ''de facto'' standard is a custom or convention that is commonly used even though its use is not required. is a Latin phrase (literally " of fact"), here meaning "in practice but not necessarily ordained by law" or "in practice or actuality, ...

for worldwide Internet routing.

Protocol classification

Routing protocol A routing protocol specifies how routers communicate with each other to distribute information that enables them to select paths between nodes on a computer network. Routers perform the traffic directing functions on the Internet; data packet ...

s may be broadly distinguished by their realm of operation in terms of network scope.

Interior gateway protocol An interior gateway protocol (IGP) or interior routing protocol is a type of routing protocol used for exchanging routing table information between gateways (commonly routers) ''within'' an autonomous system (for example, a system of corpor ...

s are used for routing within autonomous systems, while

exterior gateway protocol An exterior gateway protocol is an IP routing protocol used to exchange routing information between autonomous systems. This exchange is crucial for communications across the Internet. Notable exterior gateway protocols include Exterior Gatewa ...

s route traffic between them. The former group is exemplified by the

Routing Information Protocol The Routing Information Protocol (RIP) is one of the oldest distance-vector routing protocols which employs the hop count as a routing metric. RIP prevents routing loops by implementing a limit on the number of hops allowed in a path from so ...

(RIP) and

(OSPF), while the

Exterior Gateway Protocol An exterior gateway protocol is an IP routing protocol used to exchange routing information between autonomous systems. This exchange is crucial for communications across the Internet. Notable exterior gateway protocols include Exterior Gatewa ...

(EGP) and the

(BGP) are examples of the exterior type. BGP is the dominant route distribution protocol used on the Internet.

Routing algorithm

The IP forwarding algorithm is a specific implementation of

for

s. In order to achieve a successful transfer of data, the algorithm uses a routing table to select a next-hop router as the next destination for a

datagram A datagram is a basic transfer unit associated with a packet-switched network. Datagrams are typically structured in header and payload sections. Datagrams provide a connectionless communication service across a packet-switched network. The de ...

. The

IP address An Internet Protocol address (IP address) is a numerical label such as that is assigned to a device connected to a computer network that uses the Internet Protocol for communication. IP addresses serve two main functions: network interface i ...

of the selected router is known as the ''next-hop address.'' The IP forwarding algorithm states: : Given a destination IP address, , and network prefix, : : if ( '' matches a directly connected network address'' ) :: ''Deliver datagram to over that network link''; : else if ( ''The routing table contains a route for '' ) :: ''Send datagram to the next-hop address listed in the routing table;'' : else if ( ''a default route exists'' ) :: ''Send datagram to the default route''; : else :: ''Send a forwarding error message to the originator''; When multiple route table entries match, the entry with the longest

subnet mask A subnet, or subnetwork, is a logical subdivision of an IP network. Updated by RFC 6918. The practice of dividing a network into two or more networks is called subnetting. Computers that belong to the same subnet are addressed with an identica ...

is chosen as it is the most specific one. If there are multiple routes with the same subnet mask, the route with the lowest

metric Metric or metrical may refer to: Measuring * Metric system, an internationally adopted decimal system of measurement * An adjective indicating relation to measurement in general, or a noun describing a specific type of measurement Mathematics ...

is used. If there are multiple default routes, the metric is also used to determine which to use. If there are multiple routes with the same subnet mask and metric, the system may use equal-cost multi-path routing as a forwarding strategy. When no route is available, an ICMP error message is sent to the originator of the packet, to inform that host that the packet could not be delivered. To avoid unnecessary retransmission to avoid

network congestion Network congestion in data networking and queueing theory is the reduced quality of service that occurs when a network node or link is carrying more data than it can handle. Typical effects include queueing delay, packet loss or the blocking of ...

, the sending host should either stop transmitting or choose another address or route.

Routing table

The following presents a typical

in a Unix-like operating system:

Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
0.0.0.0         71.46.14.1      0.0.0.0         UG    0      0        0 ppp0
10.0.0.0        0.0.0.0         255.0.0.0       U     0      0        0 eth0
71.46.14.1      0.0.0.0         255.255.255.255 UH    0      0        0 ppp0
169.254.0.0     0.0.0.0         255.255.0.0     U     0      0        0 eth0
172.16.0.0      0.0.0.0         255.240.0.0     U     0      0        0 eth0
192.168.0.0     0.0.0.0         255.255.0.0     U     0      0        0 eth0
192.168.1.0     192.168.96.1    255.255.255.0   UG    0      0        0 eth0
192.168.96.0    0.0.0.0         255.255.255.0   U     0      0        0 eth0

The host has several network interfaces. ''eth0'' is the interface name of the

network interface card A network interface controller (NIC, also known as a network interface card, network adapter, LAN adapter and physical network interface) is a computer hardware component that connects a computer to a computer network. Early network interface ...

representing an

Ethernet Ethernet ( ) is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...

port. ''ppp0'' is a

PPPoE The Point-to-Point Protocol over Ethernet (PPPoE) is a network protocol for Encapsulation (networking), encapsulating Point-to-Point Protocol (PPP) frames inside Ethernet frames. It appeared in 1999, in the context of the boom of DSL as the solu ...

interface, which is configured as the

in this example. A default route is recognized by the destination ''0.0.0.0'' and the flag ''G''. A

network router A router is a computer and networking device that forwards data packets between computer networks, including internetworks such as the global Internet. Routers perform the "traffic directing" functions on the Internet. A router is connec ...

is identified by the network mask ''255.255.255.255'' and the flag ''H''.

Notes

References

{{Reflist Routing