router

What is a router?

A router is a physical or virtual appliance that passes information between two or more packet-switched computer networks. A router inspects a given data packet's destination IP address, calculates the best way for it to reach its destination and then forwards it accordingly.

A router is a common type of gateway. It is positioned where two or more networks meet at each point of presence on the internet. Hundreds of routers might forward a single packet as it moves from one network to the next on the way to its final destination. Routers exist on Layer 3, the network layer, of the Open Systems Interconnection model.

Traditional routers are standalone devices that use proprietary software. A virtual router is a software instance that performs the same functions as a physical router. Virtual routers typically run on commodity servers, either alone or packaged with other virtual network functions, such as firewall packet filtering, load balancing and WAN optimization capabilities.

Other network devices, such as wireless access points and switches, might include built-in router functionality.

How a router works

A router examines a packet header's destination IP address and compares it against a routing table to determine the packet's best next hop. Routing tables list directions for forwarding data to particular network destinations, sometimes in the context of other variables, such as cost. They amount to an algorithmic set of rules that calculate the best way to transmit traffic toward any given IP address.

A routing table often specifies a default route, which the router uses whenever it fails to find a better forwarding option for a given packet. For example, a typical home office router directs all outbound traffic along a single default route to its ISP.

Routing tables are either static or dynamic. Static routers are manually configured, while dynamic routers automatically update their routing tables based on network activity and exchange information with other devices via routing protocols.

Many routers also perform Network Address Translation (NAT), which shields the private IP addresses of a LAN by readdressing all outgoing traffic with a single shared public IP address. NAT helps to conserve globally valid IP addresses and improve network security.

Types of routers

Some of the different types of routers include the following:

• Core routers.
• Edge routers.
• Branch routers.
• Logical routers.
• Wireless routers.

Core routers

ISPs use core routers, which are the fastest and most powerful type of router. Core routers sits at the center of the internet and forward information along the main fiber optic backbone. Enterprise routers connect large organizations' networks to core routers.

Edge routers

An edge router, also known as an access router, is a lower-capacity device that resides at the boundary of a LAN and connects it to the public internet, a private WAN or an external LAN. Subscriber edge routers are edge routers used in home and small office routers.

Branch routers

Branch routers link an organization's remote office locations to its WAN, connecting to the primary campus network's edge routers. Branch routers often provide additional features, like time-division multiplexing, wireless LAN management capabilities and WAN application acceleration.

Logical routers

A logical router is a configured partition of a traditional network hardware, or physical, router. It replicates the hardware's functionality, creating multiple routing domains within a single router. Logical routers perform a subset of the tasks that physical routers can complete, and each logical router can contain multiple routing instances and routing tables.

Wireless routers

A wireless router works in the same way as the router in a hard-wired home or business LAN but enables greater mobility for notebook or portable computers. Wireless routers use the 802.11g specification, a standard that offers transmission over short distances.

Router protocols

Routing protocols determine how a router identifies other routers on the network, keeps track of all possible destinations and makes dynamic decisions for where to send each network message.

The types of routing protocols include the following:

• Open Shortest Path First. OSPF finds the best path for packets as they pass through a set of connected networks. The Internet Engineering Task Force designates OSPF as one of several Interior Gateway Protocols.
• Border Gateway Protocol. BGP manages how packets are routed across the internet through the exchange of information between edge routers. BGP offers network stability that guarantees routers can quickly adapt to send packets through another reconnection if one internet path goes down.
• Interior Gateway Routing Protocol. IGRP determines how an autonomous network exchanges routing information between gateways. Other network protocols then use the routing information to specify how transmissions should be routed.
• Enhanced Interior Gateway Routing Protocol. EIGRP evolved from IGRP. If a router can't find a route to a destination in one of these tables, it queries nearby routers, which then query routers closer to them until a route is found. When a routing table entry changes in one of the routers, it notifies nearby routers of the change instead of sending the entire table.
• Exterior Gateway Protocol. EGP determines how routing information exchanges between two neighbor gateway hosts that each have its own router. Hosts on the internet commonly use EGP to exchange routing table information.
• Routing Information Protocol. RIP is the original protocol that defines how routers should share information when traffic moves among an interconnected group of LANs. The largest number of hops allowed for RIP is 15, which limits the size of networks that RIP can support.

Editor's note: This article was reformatted to improve the reader experience.