What is a Router?
Routers operate at Layer3 of the OSI Layer and are categorized in the Network Layer. A router can be programmed to find the best routes between networks. The router can traverse many networks and is capable of choosing the best route to the destination. Offices in different and remote locations can be connected together using routers using dedicated or switched lines. The lines are usually provided by telephone companies or Internet Service Providers (ISPs). The routers connect to lines using serial interfaces which usually come installed in the device. Connecting the lines to the serial interfaces on the router allow for a Wider Area Network (WAN) connection.
For example, if you have one office in New York and other in London then connecting in the above way will allow for the sending of data and voice transmissions. The Internet is made up of thousands of routers and this allows people all over the globe to communicate using for example, email. Routers by default do not send broadcasts as it is routing data.
If wires are used, each computer is connected by its own wire to the router. Modern wired-only routers designed for the home or small business typically have one “input” port (to the Internet) and four “output” ports, one or more of which can be connected to other computers. A typical modern home wireless router, in addition to having four wired ports, also allows several devices to connect with it wirelessly. Most modern personal computers are built with a wired port (almost always an Ethernet type), which allows them to connect to a router with the addition of just a cable (typically a Category type). To connect with a wireless router, a device must have an adapter. This is sometimes, but not always, included with the computer at manufacture. Some electronic games, including handheld electronic games, have an adapter built-in, or one can be added later.
More technically, a router is a networking device whose software and hardware are usually tailored to the tasks of routing and forwarding information. Routers connect two or more logical subnets, which do not necessarily map one-to-one to the physical interfaces of the router. The term “layer 3 switching” is often used interchangeably with routing, but switch is a general term without a rigorous technical definition. In marketing usage, a switch is generally optimized for Ethernet LAN interfaces and may not have other physical interface types.
In comparison, the network hub (predecessor of the “switch” or “switching hub”) does not do any routing, instead every packet it receives on one network line gets forwarded to all the other network lines.
Routers operate in two different planes:
Control plane, in which the router learns the outgoing interface that is most appropriate for forwarding specific packets to specific destinations,
Forwarding plane, which is responsible for the actual process of sending a packet received on a logical interface to an outbound logical interface.
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 CiscoCRS-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 and internal use
Router Cisco Wireless
Routers intended for ISP and major enterprise connectivity will almost invariably exchange routing information with the Border Gateway Protocol (BGP). RFC 4098 defines several types of BGP-speaking routers:
Edge Router – Placed at the edge of an ISP network, it speaks external BGP (EBGP) to a BGP speaker in another provider or large enterprise Autonomous System(AS).
Subscriber Edge Router – 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.
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 router that resides within the middle or backbone of the LAN network rather than at its periphery.
Within an ISP – Internal to the provider’s AS, such a router speaks internal BGP (IBGP) to that provider’s edge routers, other intra-provider core routers, or the provider’s inter-provider border routers.
“Internet backbone” – The Internet does not have a clearly identifiable backbone, as did its predecessors. See default-free zone(DFZ). Nevertheless, it is the major ISPs’ routers that make up what many would consider the core. These ISPs operate all four types of the BGP-speaking routers described here. In ISP usage, a “core” router is internal to an ISP, and 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 (MPLS).
Routers are also used for port forwarding for private servers.
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