Routers

Routers act as network layer relays between networks.

While bridges are normally restricted to connecting LANs within an internetwork,

routers have the capability of connecting networks of different types

A router participates as a device on each attached network and exchanges information with devices on those networks. These are end node capabilities.

A router has the additional capability of exchanging information with other routers and with end nodes on remote networks as long as they support the same network layer protocol.

Therefore, routers are protocol-dependent, unlike bridges.

Packet

Packets (also known as frames) are defined as chunk data which has been packaged, addressed, and sent into the network towards its destination much as a letter is placed into an envelope, addressed, and dropped into a mailbox.

The data may represent an entire message or a segment of a message, depending on media or device limitations on the amount of information that may be enveloped.

Routers forward packets based on information held in the network layer headers of packets they receive on their attached networks. Network layer headers contain address information (typically consisting of a network identifier and a host identifier) and associated control to allow the packets to be routed to their correct destination. Packets that have no network layer header, or that have a network layer header for a protocol not supported by a particular router, are discarded.

There are a number of network layer protocols, each of which has its own network layer header format for addressing and control.

A router must be configured with the network layer address of each of its network connections. When it receives an incoming frame with a compatible network layer header, it determines whether the destination address is on the same network. If it is, then the frame is discarded. Otherwise, the router forwards the frame to the destination device (if on a network attached to the router), or to the next router in the path to the destination device.

In order to do this, a router must maintain routing tables containing information about the next router in the path to every reachable destination in the internetwork. The process for doing this includes two stages: 

• It must acquire route information.
  
• It must determine the best routes to insert into its routing table.
  

Route information can be acquired by manual configuration (these are called static routes), or can be learned automatically from other routers using routing table maintenance protocols (these are called dynamic routes)