1.1 Router

Major network component : router, switch, hub, and bridge.

A router is a device that interconnects two or more computer networks. Basicly, function of the router are to interconnecting networks by selecting best path (routing/path selection) for a packet to travel, and forward packet (packet switching) to outgoing interface using best path in routing table. Routing or selecting best path in a router is only based on destination layer 3 address (IP), although this decision can be altered using source interface or source addresses, this method named Policy Based Routing (hopefully we can discuss this topic later).

A router operates in two operational plane (sub system) in order to accomplish those two basic functions of the router :

• 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.

As an additional feature, routers are capable of providing filtering on a network address and application port level (Access List).

Routers use destination IP address to forward packets, the path a packet takes is determined after a router consults information in the routing table. After router determines the best path, packet is encapsulated into a frame, frame is then placed on network medium in form of Bits. Vise versa proccess apply for de-encapsulation proccess. This (de-)encapsulation proccess will discuss more deeper in OSI 7 layer section.