
A router is a
networking device
Networking hardware, also known as network equipment or computer networking devices, are electronic devices which are required for communication and interaction between devices on a computer network. Specifically, they mediate data transmission in ...
that forwards
data packets between
computer network
A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections ar ...
s.
Routers perform the traffic directing functions between networks and on the global
Internet
The Internet (or internet) is the 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'' that consists ...
. Data sent through a network, such as a
web page or
email
Electronic mail (email or e-mail) is a method of exchanging messages ("mail") between people using electronic devices. Email was thus conceived as the electronic ( digital) version of, or counterpart to, mail, at a time when "mail" mean ...
, is in the form of
data packets. A packet is typically
forwarded from one router to another router through the networks that constitute an
internetwork (e.g. the Internet) until it reaches its destination
node
In general, a node is a localized swelling (a "knot") or a point of intersection (a vertex).
Node may refer to:
In mathematics
* Vertex (graph theory), a vertex in a mathematical graph
* Vertex (geometry), a point where two or more curves, line ...
.
A router is connected to two or more data lines from different
IP networks. When a data packet comes in on one of the lines, the router reads the
network address information in the packet header to determine the ultimate destination. Then, using information in its
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 with t ...
or
routing policy
A routing protocol specifies how routers communicate with each other to distribute information that enables them to select routes between nodes on a computer network. Routers perform the traffic directing functions on the Internet; data packets ...
, it directs the packet to the next network on its journey.
The most familiar type of IP routers are
home and small office routers that simply forward
IP packets between the home computers and the Internet. More sophisticated routers, such as enterprise routers, connect large business or ISP networks up to the powerful
core routers that forward data at high speed along the
optical fiber
An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparency and translucency, transparent fiber made by Drawing (manufacturing), drawing glass (silica) or plastic to a diameter slightly thicker than that of a Hair ...
lines of the
Internet backbone.
Routers can be built from standard computer parts but are mostly
specialized purpose-built computers. Early routers used
software
Software is a set of computer programs and associated software documentation, documentation and data (computing), data. This is in contrast to Computer hardware, hardware, from which the system is built and which actually performs the work.
...
-based forwarding, running on a
CPU
A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, and ...
. More sophisticated devices use
application-specific integrated circuit
An application-specific integrated circuit (ASIC ) is an integrated circuit (IC) chip customized for a particular use, rather than intended for general-purpose use, such as a chip designed to run in a digital voice recorder or a high-effici ...
s (ASICs) to increase performance or add advanced filtering and
firewall functionality.
Operation
When multiple routers are used in interconnected networks, the routers can exchange information about destination addresses using a
routing protocol
A routing protocol specifies how routers communicate with each other to distribute information that enables them to select routes between nodes on a computer network. Routers perform the traffic directing functions on the Internet; data packet ...
. Each router builds up 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 with t ...
, a list of routes, between two computer systems on the interconnected networks.
The software that runs the router is composed of two functional processing units that operate simultaneously, called ''planes'':
*
Control plane: A router maintains a routing table that lists which route should be used to forward a data packet, and through which physical interface connection. It does this using internal pre-configured directives, called
static route Static routing is a form of routing that occurs when a router uses a manually-configured routing entry, rather than information from dynamic routing traffic. In many cases, static routes are manually configured by a network administrator by adding ...
s, or by learning routes
dynamically using a routing protocol. Static and dynamic routes are stored in the routing table. The control-plane logic then strips non-essential directives from the table and builds a
forwarding information base (FIB) to be used by the forwarding plane.
*
Forwarding plane: This unit forwards the data packets between incoming and outgoing interface connections. It reads the
header of each packet as it comes in, matches the destination to entries in the FIB supplied by the control plane, and directs the packet to the outgoing network specified in the FIB.
Applications

A router may have interfaces for multiple types of
physical layer
In the seven-layer OSI model of computer networking, the physical layer or layer 1 is the first and lowest layer; The layer most closely associated with the physical connection between devices. This layer may be implemented by a PHY chip.
Th ...
connections, such as copper cables,
fiber optic
An optical fiber, or optical fibre in Commonwealth English, is a flexible, transparency and translucency, transparent fiber made by Drawing (manufacturing), drawing glass (silica) or plastic to a diameter slightly thicker than that of a Hair ...
, or
wireless
Wireless communication (or just wireless, when the context allows) is the transfer of information between two or more points without the use of an electrical conductor, optical fiber or other continuous guided medium for the transfer. The mos ...
transmission. It can also support multiple
network layer
In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate routers.
Functions
The network layer provides the means of trans ...
transmission standards. Each network interface is used to enable data packets to be forwarded from one transmission system to another. Routers may also be used to connect two or more logical groups of computer devices known as
subnets, each with a unique
network prefix.
Routers may provide connectivity within enterprises, between enterprises and the Internet, or between
internet service provider
An Internet service provider (ISP) is an organization that provides services for accessing, using, or participating in the Internet. ISPs can be organized in various forms, such as commercial, community-owned, non-profit, or otherwise privatel ...
s' (ISPs') networks. The largest routers (such as the
Cisco CRS-1 or
Juniper PTX) interconnect the various ISPs, or may be used in large enterprise networks. Smaller routers usually provide connectivity for typical home and office networks.
All sizes of routers may be found inside enterprises.
The most powerful routers are usually found in ISPs, academic and research facilities. Large businesses may also need more powerful routers to cope with ever-increasing demands of
intranet data traffic. A
hierarchical internetworking model for interconnecting routers in large networks is in common use.
Access, core and distribution

Access routers, including
small office/home office
Small office/home office (or single office/home office; sometimes short SOHO) refers to the category of business or cottage industry that involves from 1 to 10 workers.
In New Zealand, the Ministry of Business, Innovation and Employment (MBI ...
(SOHO) models, are located at home and customer sites such as branch offices that do not need
hierarchical routing of their own. Typically, they are optimized for low cost. Some SOHO routers are capable of running alternative free Linux-based firmware like
Tomato
The tomato is the edible berry of the plant ''Solanum lycopersicum'', commonly known as the tomato plant. The species originated in western South America, Mexico, and Central America. The Mexican Nahuatl word gave rise to the Spanish word , ...
,
OpenWrt
OpenWrt (from ''open wireless router'') is an open-source project for embedded operating systems based on Linux, primarily used on embedded devices to route network traffic. The main components are Linux, util-linux, musl, and BusyBox. A ...
, or
DD-WRT.
Distribution routers aggregate traffic from multiple access routers. Distribution routers are often responsible for enforcing quality of service across a
wide area network
A wide area network (WAN) is a telecommunications network that extends over a large geographic area. Wide area networks are often established with leased telecommunication circuits.
Businesses, as well as schools and government entities, u ...
(WAN), so they may have considerable memory installed, multiple WAN interface connections, and substantial onboard data processing routines. They may also provide connectivity to groups of file servers or other external networks.
In enterprises, a
core router may provide a
collapsed backbone interconnecting the distribution tier routers from multiple buildings of a campus, or large enterprise locations. They tend to be optimized for high bandwidth, but lack some of the features of edge routers.
Security
External networks must be carefully considered as part of the overall security strategy of the local network. A router may include a
firewall,
VPN handling, and other security functions, or they may be handled by separate devices. Routers also commonly perform
network address translation which restricts connections initiated from external connections but is not recognized as a security feature by all experts. Some experts argue that
open source
Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use the source code, design documents, or content of the product. The open-source model is a decentralized sof ...
routers are more secure and reliable than
closed source
Proprietary software is software that is deemed within the free and open-source software to be non-free because its creator, publisher, or other rightsholder or rightsholder partner exercises a legal monopoly afforded by modern copyright and in ...
routers because open-source routers allow mistakes to be quickly found and corrected.
Routing different networks
Routers are also often distinguished on the basis of the network in which they operate. A router in a
local area network (LAN) of a single organisation is called an ''interior router''. A router that is operated in the
Internet
The Internet (or internet) is the 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'' that consists ...
backbone is described as ''exterior router''. While a router that connects a LAN with the Internet or a
wide area network
A wide area network (WAN) is a telecommunications network that extends over a large geographic area. Wide area networks are often established with leased telecommunication circuits.
Businesses, as well as schools and government entities, u ...
(WAN) is called a ''border router'', or ''
gateway router''.
Internet connectivity and internal use
Routers intended for ISP and major enterprise connectivity usually exchange routing information using the
Border Gateway Protocol (BGP). defines the types of BGP routers according to their functions:
* ''Edge router'' (also called a ''provider edge router''): Placed at the edge of an ISP network. The router uses
Exterior Border Gateway Protocol (EBGP) to routers at other ISPs or large enterprise
autonomous system Autonomous system may refer to:
* Autonomous system (Internet), a collection of IP networks and routers under the control of one entity
* Autonomous system (mathematics), a system of ordinary differential equations which does not depend on the inde ...
s.
* ''Subscriber edge router'' (also called a ''customer edge router''): Located at the edge of the subscriber's network, it also uses EBGP to its provider's autonomous system. It is typically used in an (enterprise) organization.
* ''Inter-provider border router'': A BGP router for interconnecting ISPs that maintains BGP sessions with other BGP routers in ISP Autonomous Systems.
*
Core router: Resides within an Autonomous System as a backbone to carry traffic between edge routers.
* Within an ISP: In the ISP's autonomous system, a router uses internal BGP to communicate with other ISP edge routers, other
intranet core routers, or the ISP's intranet provider border routers.
* Internet backbone: The Internet no longer has a clearly identifiable backbone, unlike its predecessor networks. See
default-free zone (DFZ). The major ISPs' system routers make up what could be considered to be the current Internet backbone core. ISPs operate all four types of the BGP routers described here. An ISP core router is used to interconnect its edge and border routers. Core routers may also have specialized functions in
virtual private networks based on a combination of BGP and
Multi-Protocol Label Switching protocols.
* Port forwarding: Routers are also used for
port forwarding between private Internet-connected servers.
[
* Voice, data, fax, and video processing routers: Commonly referred to as access servers or gateways, these devices are used to route and process voice, data, video and fax traffic on the Internet. Since 2005, most long-distance phone calls have been processed as IP traffic (]VOIP
Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms Interne ...
) through a voice gateway. Use of access server-type routers expanded with the advent of the Internet, first with dial-up access and another resurgence with voice phone service.
* Larger networks commonly use multilayer switches, with layer-3 devices being used to simply interconnect multiple subnets within the same security zone, and higher-layer switches when filtering, translation
Translation is the communication of the Meaning (linguistic), meaning of a #Source and target languages, source-language text by means of an Dynamic and formal equivalence, equivalent #Source and target languages, target-language text. The ...
, load balancing, or other higher-level functions are required, especially between zones.
History
The concept of an ''Interface computer'' was first proposed by Donald Davies
Donald Watts Davies, (7 June 1924 – 28 May 2000) was a Welsh computer scientist who was employed at the UK National Physical Laboratory (NPL).
In 1965 he conceived of packet switching, which is today the dominant basis for data communic ...
for the NPL network in 1966. The same idea was conceived by Wesley Clark
Wesley Kanne Clark (born December 23, 1944) is a retired United States Army officer. He graduated as valedictorian of the class of 1966 at West Point and was awarded a Rhodes Scholarship to the University of Oxford, where he obtained a degree ...
the following year for use in the ARPANET
The Advanced Research Projects Agency Network (ARPANET) was the first wide-area packet-switched network with distributed control and one of the first networks to implement the TCP/IP protocol suite. Both technologies became the technical foun ...
. Named ''Interface Message Processors'' (IMPs), these computers had fundamentally the same functionality as a router does today. The idea for a router (called '' gateway'' at the time) initially came about through an international group of computer networking researchers called the International Networking Working Group (INWG). Set up in 1972 as an informal group to consider the technical issues involved in connecting different networks, it became a subcommittee of the International Federation for Information Processing
The International Federation for Information Processing (IFIP) is a global organisation for researchers and professionals working in the field of computing to conduct research, develop standards and promote information sharing.
Established in 196 ...
later that year. These gateway devices were different from most previous packet switching
In telecommunications, packet switching is a method of grouping Data (computing), data into ''network packet, packets'' that are transmitted over a digital Telecommunications network, network. Packets are made of a header (computing), header and ...
schemes in two ways. First, they connected dissimilar kinds of networks, such as serial lines and local area networks. Second, they were connectionless devices, which had no role in assuring that traffic was delivered reliably, leaving that function entirely to the hosts. This particular idea, the end-to-end principle, had been previously pioneered in the CYCLADES
The Cyclades (; el, Κυκλάδες, ) are an island group in the Aegean Sea, southeast of mainland Greece and a former administrative prefecture of Greece. They are one of the island groups which constitute the Aegean archipelago. The na ...
network.
The idea was explored in more detail, with the intention to produce a prototype system as part of two contemporaneous programs. One was the initial DARPA
The Defense Advanced Research Projects Agency (DARPA) is a research and development agency of the United States Department of Defense responsible for the development of emerging technologies for use by the military.
Originally known as the Ad ...
-initiated program, which created the TCP/IP architecture in use today. The other was a program at Xerox PARC to explore new networking technologies, which produced the PARC Universal Packet system; due to corporate intellectual property concerns it received little attention outside Xerox for years. Some time after early 1974, the first Xerox routers became operational. The first true IP router was developed by Ginny Strazisar at BBN, as part of that DARPA-initiated effort, during 1975–1976. By the end of 1976, three PDP-11
The PDP-11 is a series of 16-bit minicomputers sold by Digital Equipment Corporation (DEC) from 1970 into the 1990s, one of a set of products in the Programmed Data Processor (PDP) series. In total, around 600,000 PDP-11s of all models were so ...
-based routers were in service in the experimental prototype Internet.
The first multiprotocol routers were independently created by staff researchers at MIT and Stanford
Stanford University, officially Leland Stanford Junior University, is a private research university in Stanford, California. The campus occupies , among the largest in the United States, and enrolls over 17,000 students. Stanford is consider ...
in 1981 and both were also based on PDP-11s. Stanford's router program was by William Yeager
William "Bill" Yeager (born June 16, 1940, San Francisco) is an American engineer. He is best known for being the inventor of a packet-switched, "Ships in the Night," multiple- protocol router in 1981, during his 20-year tenure at Stanford's ...
and MIT's by Noel Chiappa. Virtually all networking now uses TCP/IP, but multiprotocol routers are still manufactured. They were important in the early stages of the growth of computer networking when protocols other than TCP/IP were in use. Modern routers that handle both IPv4 and IPv6 are multiprotocol but are simpler devices than ones processing AppleTalk, DECnet, IP, and Xerox protocols.
From the mid-1970s and in the 1980s, general-purpose minicomputer
A minicomputer, or colloquially mini, is a class of smaller general purpose computers that developed in the mid-1960s and sold at a much lower price than mainframe and mid-size computers from IBM and its direct competitors. In a 1970 survey, ...
s served as routers. Modern high-speed routers are network processors or highly specialized computers with extra hardware acceleration
Hardware acceleration is the use of computer hardware designed to perform specific functions more efficiently when compared to software running on a general-purpose central processing unit (CPU). Any transformation of data that can be calc ...
added to speed both common routing functions, such as packet forwarding, and specialized functions such as IPsec
In computing, Internet Protocol Security (IPsec) is a secure network protocol suite that authenticates and encrypts packets of data to provide secure encrypted communication between two computers over an Internet Protocol network. It is used in ...
encryption. There is substantial use of Linux
Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, which i ...
and Unix
Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...
software-based machines, running open source
Open source is source code that is made freely available for possible modification and redistribution. Products include permission to use the source code, design documents, or content of the product. The open-source model is a decentralized sof ...
routing code, for research and other applications. The Cisco IOS operating system was independently designed. Major router operating systems, such as Junos
Junos OS (also known as Juniper Junos, Junos and JUNOS) is a FreeBSD-based network operating system used in Juniper Networks routing, switching and security devices.
Versioning
Junos OS was first made available on 7 July 1998, with new feature ...
and NX-OS, are extensively modified versions of Unix software.
Forwarding
The main purpose of a router is to connect multiple networks and forward packets destined either for directly attached networks or more remote networks. A router is considered a layer-3 device because its primary forwarding decision is based on the information in the layer-3 IP packet, specifically the destination IP address. When a router receives a packet, it searches its routing table to find the best match between the destination IP address of the packet and one of the addresses in the routing table. Once a match is found, the packet is encapsulated in the layer-2 data link frame for the outgoing interface indicated in the table entry. A router typically does not look into the packet payload, but only at the layer-3 addresses to make a forwarding decision, plus optionally other information in the header for hints on, for example, quality of service
Quality of service (QoS) is the description or measurement of the overall performance of a service, such as a telephony or computer network, or a cloud computing service, particularly the performance seen by the users of the network. To quantitat ...
(QoS). For pure IP forwarding, a router is designed to minimize the state
State may refer to:
Arts, entertainment, and media Literature
* ''State Magazine'', a monthly magazine published by the U.S. Department of State
* ''The State'' (newspaper), a daily newspaper in Columbia, South Carolina, United States
* '' Our ...
information associated with individual packets. Once a packet is forwarded, the router does not retain any historical information about the packet.
The routing table itself can contain information derived from a variety of sources, such as a default
Default may refer to:
Law
* Default (law), the failure to do something required by law
** Default (finance), failure to satisfy the terms of a loan obligation or failure to pay back a loan
** Default judgment, a binding judgment in favor of ei ...
or static route Static routing is a form of routing that occurs when a router uses a manually-configured routing entry, rather than information from dynamic routing traffic. In many cases, static routes are manually configured by a network administrator by adding ...
s that are configured manually, or dynamic entries from routing protocol
A routing protocol specifies how routers communicate with each other to distribute information that enables them to select routes between nodes on a computer network. Routers perform the traffic directing functions on the Internet; data packet ...
s where the router learns routes from other routers. A default route is one that is used to route all traffic whose destination does not otherwise appear in the routing table; it is common – even necessary – in small networks, such as a home or small business where the default route simply sends all non-local traffic to the Internet service provider
An Internet service provider (ISP) is an organization that provides services for accessing, using, or participating in the Internet. ISPs can be organized in various forms, such as commercial, community-owned, non-profit, or otherwise privatel ...
. The default route can be manually configured (as a static route); learned by dynamic routing protocols; or be obtained by DHCP.
A router can run more than one routing protocol at a time, particularly if it serves as an autonomous system border router between parts of a network that run different routing protocols; if it does so, then redistribution may be used (usually selectively) to share information between the different protocols running on the same router.
Besides deciding to which interface a packet is forwarded, which is handled primarily via the routing table, a router also has to manage congestion when packets arrive at a rate higher than the router can process. Three policies commonly used are tail drop, random early detection (RED), and weighted random early detection (WRED). Tail drop is the simplest and most easily implemented: the router simply drops new incoming packets once buffer space in the router is exhausted. RED probabilistically drops datagrams early when the queue exceeds a pre-configured portion of the buffer, until reaching a pre-determined maximum, when it drops all incoming packets, thus reverting to tail drop. WRED can be configured to drop packets more readily dependent on the type of traffic.
Another function a router performs is traffic classification and deciding which packet should be processed first. This is managed through QoS, which is critical when Voice over IP
Voice over Internet Protocol (VoIP), also called IP telephony, is a method and group of technologies for the delivery of voice communications and multimedia sessions over Internet Protocol (IP) networks, such as the Internet. The terms Interne ...
is deployed, so as not to introduce excessive latency.
Yet another function a router performs is called policy-based routing where special rules are constructed to override the rules derived from the routing table when a packet forwarding decision is made.
Some of the functions may be performed through an application-specific integrated circuit
An application-specific integrated circuit (ASIC ) is an integrated circuit (IC) chip customized for a particular use, rather than intended for general-purpose use, such as a chip designed to run in a digital voice recorder or a high-effici ...
(ASIC) to avoid overhead of scheduling CPU time to process the packets. Others may have to be performed through the CPU as these packets need special attention that cannot be handled by an ASIC.
See also
* Mobile broadband modem
* Modem
A modulator-demodulator or modem is a computer hardware device that converts data from a digital format into a format suitable for an analog transmission medium such as telephone or radio. A modem transmits data by modulating one or more c ...
* Residential gateway
* Switch virtual interface
* Wireless router
Notes
References
External links
{{Authority control
Internet architecture
Hardware routers
Networking hardware
Server appliance
Computer networking