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A router is a device that interconnects two or more computer networks, and selectively interchanges packets of data between them. Each data packet contains address information that a router can use to determine if the source and destination are on the same network, or if the data packet must be transferred from one network to another. Where multiple routers are used in a large collection of interconnected networks, the routers exchange information about target system addresses, so that each router can build up a table showing the preferred paths between any two systems on the interconnected networks.

A router is a networking device whose software and hardware are customized to the tasks of routing and forwarding information. A router has two or more network interfaces, which may be to different physical types of network (such as copper cables, fiber, or wireless) or different network standards. Each network interface is a specialized device that converts electric signals from one form to another.

Routers connect two or more logical subnets, which do not share a common network address. The subnets in the router do not necessarily map one-to-one to the physical interfaces of the router. The term "layer 3 switching" is used often interchangeably with the term "routing". The term switching is generally used to refer to data forwarding between two network devices that share a common network address. This is also called layer 2 switching or LAN switching.

Conceptually, a router operates in two operational planes (or sub-systems):

• Control plane: where a router builds a table (called routing table) as how a packet should be forwarded through which interface, by using either statically configured statements (called static routes) or by exchanging information with other routers in the network through a dynamical routing protocol;
• Forwarding plane: where the router actually forwards traffic (called packets in IP) from ingress (incoming) interfaces to an egress (outgoing) interface that is appropriate for the destination address that the packet carries with it, by following rules derived from the routing table that has been built in the control plane.

Types of routers
Routers may provide connectivity inside enterprises, between enterprises and the Internet, and inside internet service providers (ISPs). The largest routers (for example the Cisco CRS-1 or Juniper T1600) interconnect ISPs, are used inside ISPs, or may be used in very large enterprise networks. The smallest routers provide connectivity for small and home offices.


Routers for Internet connectivity & internal useRouters intended for ISP and major enterprise connectivity almost invariably exchange routing information using the Border Gateway Protocol (BGP). RFC 4098 defines several types of BGP-speaking routers according to the routers' functions:

• Edge router (ER): An ER is placed at the edge of an ISP network. The router speaks external BGP (EBGP) to a BGP speaker in another provider or large enterprise Autonomous System(AS). This type of router is also called PE (Provider Edge) routers.
• Subscriber edge router (SER): An SER is located at the edge of the subscriber's network, it speaks EBGP to its provider's AS(s). It belongs to an end user (enterprise) organization. This type of router is also called CE (Customer Edge) routers.
• Inter-provider border router: Interconnecting ISPs, this is a BGP-speaking router that maintains BGP sessions with other BGP speaking routers in other providers' ASes.
• Core router: A core router is one that resides within an AS as back bone to carry traffic between edge routers.

Routers are also used for port forwarding for private servers.

The very first device that had fundamentally the same functionality as a router does today, i.e a packet switch, was the Interface Message Processor (IMP); IMPs were the devices that made up the ARPANET, the first packet switching network. The idea for a router (although they were called "gateways" at the time) initially came about through an international group of computer networking researchers called the International Network Working Group (INWG). Set up in 1972 as an informal group to consider the technical issues involved in connecting different networks, later that year it became a subcommittee of the International Federation for Information Processing.

These devices were different from most previous packet switches 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 entirely to the hosts (although this particular idea had been previously pioneered in the CYCLADES network).

The idea was explored in more detail, with the intention to produce a real prototype system, as part of two contemporaneous programs. One was the initial DARPA-initiated program, which created the TCP/IP architecture of today. The other was a program at Xerox PARC to explore new networking technologies, which produced the PARC Universal Packet system, although due to corporate intellectual property concerns it received little attention outside Xerox until years later.

The earliest Xerox routers came into operation sometime after early 1974. The first true IP router was developed by Virginia Strazisar at BBN, as part of that DARPA-initiated effort, during 1975-1976. By the end of 1976, three PDP-11-based routers were in service in the experimental prototype Internet.

The first multiprotocol routers were independently created by staff researchers at MIT and Stanford in 1981; the Stanford router was done by William Yeager, and the MIT one by Noel Chiappa; both were also based on PDP-11s.

As virtually all networking now uses IP at the network layer, multiprotocol routers are largely obsolete, although they were important in the early stages of the growth of computer networking, when several protocols other than TCP/IP were in widespread use. Routers that handle both IPv4 and IPv6 arguably are multiprotocol, but in a far less variable sense than a router that processed AppleTalk, DECnet, IP, and Xerox protocols.

In the original era of routing (from the mid-1970s through the 1980s), general-purpose mini-computers served as routers. Although general-purpose computers can perform routing, modern high-speed routers are highly specialized computers, generally with extra hardware added to accelerate both common routing functions such as packet forwarding and specialised functions such as IPsec encryption.

Still, there is substantial use of Linux and Unix machines, running open source routing code, for routing research and selected other applications. While Cisco's operating system was independently designed, other major router operating systems, such as those from Juniper Networks and Extreme Networks, are extensively modified but still have Unix ancestry.

Enterprise routers

All sizes of routers may be found inside enterprises. The most powerful routers tend to be found in ISPs and academic & research facilities. Large businesses may also need powerful routers.

A three-layer model is in common use, not all of which need be present in smaller networks.

Access routers, including 'small office/home office' (SOHO) models, are located at customer sitesoptimized for low cost. Some SOHO routers are capable of running alternative free Linux-based firmwares like OpenWrt. such as branch offices that do not need hierarchical routing of their own. Typically, they are

Distribution routers aggregate traffic from multiple access routers, either at the same site, or to collect the data streams from multiple sites to a major enterprise location. Distribution routers often are responsible for enforcing quality of service (QoS) across a WAN, so they may have considerable memory, multiple WAN interfaces, and substantial processing intelligence.

They may also provide connectivity to groups of servers or to external networks. In the latter application, the router's functionality must be carefully considered as part of the overall security architecture. Separate from the router may be a firewall or VPN concentrator, or the router may include these and other security functions.

When an enterprise is widely distributed with no central location(s), the function of core routing may be subsumed by the WAN service to which the enterprise subscribes, and the distribution routers become the highest tier.

For pure Internet Protocol (IP) forwarding function, a router is designed to minimize the state information on individual packets. A router does not look into the actual data contents that the packet carries, but only at the layer 3 addresses to make a forwarding decision, plus optionally other information in the header for hint on, for example, QoS. Once a packet is forwarded, the router does not retain any historical information about the packet, but the forwarding action can be collected into the statistical data, if so configured.

Forwarding decisions can involve decisions at layers other than the IP internetwork layer or OSI layer 3. A function that forwards based on data link layer, or OSI layer 2, information, is properly called a bridge or switch. This function is referred to as layer 2 switching, as the addresses it uses to forward the traffic are layer 2 addresses in the OSI layer model.

Besides making decision as which interface a packet is forwarded to, 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 in the Internet are tail drop, random early detection, and weighted random early detection. Tail drop is the simplest and most easily implemented; the router simply drops packets once the length of the queue exceeds the size of the buffers in the router. Random early detection (RED) probabilistically drops datagrams early when the queue is about to exceed a pre-configured size of the queue. Weighted random early detection requires a weight on the average queue size to act upon when the traffic is about to exceed the pre-configured size, so that short bursts will not trigger random drops.

Another function a router performs is to decide which packet should be processed first when multiple queues exist. This is managed through QoS (Quality of Service), which is critical when VoIP (Voice over IP) is deployed, so that delays between packets do not exceed 150ms to maintain the quality of voice conversations.

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 packet forwarding decision is made.

These functions may or may not be performed through the same internal paths that the packets travel inside the router. Some of the functions may be performed through an ASIC (application-specific integrated circuit) to avoid overhead caused by multiple CPU cycles, and others may have to be performed through the CPU as these packets need special attention that cannot be handled by an ASIC.

The major router manufacturers include:

• Alcatel-Lucent
• Asus
• Avaya
• Belkin
• Brocade
• Buffalo
• Cisco Systems
• D-link
• Extreme Networks
• Fujitsu
• Huawei
• Juniper Networks
• Netgear
• TP-Link
• UTStarcom
• ZTE
• ZyXEL