Router

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This article is about a computer networking device. For the kind of rotating cutting tool called wood router, see wood router.

A rack-mounted Cisco 7200 router in use in an ISP's data center

A router (IPA pronunciation: [raʊtɚ] is a computer networking device that forwards data packets across a network toward their destinations, through a process known as routing. Routing occurs at Layer 3 (the network layer i.e. Internet Protocol (IP)) of the OSI seven-layer protocol stack.

1 Function
2 Types of routers
3 Manufacturers of routers
4 "Software" routers
5 See also
6 External links

[edit] Function

Routers are like intersections whereas switches are like streets

A router acts as a junction between two or more networks to transfer data packets among them. A router is different from a switch. A switch connects devices to form a local area network (LAN).

One easy illustration for the different functions of routers and switches is to think of switches as neighborhood streets, and the router as the intersections with the street signs. Each house on the street has an address within a range on the block. In the same way, a switch connects various devices each with their own IP address(es) on a LAN.

However, the switch knows nothing about IP addresses except its own management address. Routers connect networks together the way that on-ramps or major intersections connect streets to both highways and freeways, etc. The street signs at the intersection (routing table) show which way the packets need to flow.

So for example, a router at home connects the Internet service provider's (ISP) network (usually on an Internet address) together with the LAN in the home (typically using a range of private IP addresses, see network address translation (NAT)) and a single broadcast domain. The switch connects devices together to form the LAN. Sometimes the switch and the router are combined together in one single package sold as a multiple port router.

In order to route packets, a router communicates with other routers using routing protocols and using this information creates and maintains a routing table. The routing table stores the best routes to certain network destinations, the "routing metrics" associated with those routes, and the path to the next hop router. See the routing article for a more detailed discussion of how this works.

Routing is most commonly associated with Internet Protocol(IP), although other less-popular routed protocols are in use.

[edit] Types of routers

A basic home router connects an ethernet network to a broadband connection. These routers will typically include connectors for both networks (13, 17), along with a basic processor (4), RAM (6), and flash memory (7).

The setup menu for a typical consumer router.

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

Other changes also improve reliability, such as using DC power rather than line power (which can be provided from batteries in data centers), and using solid state rather than magnetic storage for program loading. Large modern routers have thus come to resemble telephone switches, with whose technology they are currently converging and may eventually replace. Small routers have become a common household item.

A router that connects clients to the Internet is called an edge router. A router that serves solely to transmit data between other routers, e.g. inside the network of a ISP, is called a core router.

A router is normally used to connect at least two networks, but a special variety of router is the one-armed router, used to route packets in a virtual LAN environment. In the case of a one-armed router, the multiple attachments to different networks are all over the same physical link.

In mobile ad-hoc networks every host performs routing and forwarding by itself, while in wired networks there is usually just one router for a whole broadcast domain.

In recent times many routing functions have been added to LAN switches (a marketing term for high-speed bridges), creating "Layer 2/3 switches" which route traffic at near wire speed.

Routers are also now being implemented as Internet gateways, primarily for small networks like those used in homes and small offices. This application is mainly where the Internet connection is an always-on broadband connection like cable modem or DSL. These are routers in the true sense because they join two networks together - the WAN and the LAN – and have a routing table. Often these small routers support the RIP protocol, although in a home application the routing function does not serve much purpose since there are only two ways to go - the WAN and the LAN. In addition, these routers typically provide DHCP, NAT, DMZ and firewall services. Sometimes these routers can provide content filtering and VPN. Typically they are used in conjunction with either a cable or DSL modem, but that function can also be built-in.

Larger routers are typically found in data centers. They join multiple networks together with a lot of bandwidth. Depending on their function, these routers will support any number of routing protocols including IS-IS OSPF IGRP EIGRP RIP BGP and EGP.

[edit] Manufacturers of routers

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There are a number of manufacturers of routers including:

[edit] "Software" routers

With the proper software and 2 or more network cards, ordinary PCs, even old ones, can be made into routers.

Most Unix-like operating systems include all necessary software to perform routing:

LEAF Project
Coyote Linux
FREESCO
GNU Zebra
SmoothWall
m0n0wall
FreeBSD
NetBSD
OpenBSD
IPCop
The Linux Router Project
fdgw
ClarkConnect
Vyatta OFR which is a small distribution based on XORP
ZeroShell a small Linux distribution which is able to act as router, bridge and VPN box

The list includes only some examples that specialise in routing. NAT has been a built-in function of the Linux kernel since version 1.3. Originally an option that needed to be compiled into the kernel, most modern versions have NAT as a module that merely needs to be turned on. See List of Linux distributions, BSD, Unix-like for more.