There are many devices and technologies working together to enable a network. The primary device for networking is the router. The routers connect multiple networks together. Communication between different networks is not possible without a routers. The main functions of the router are to decide the best path to the destination and send traffic to the next router along that path to the destination.
When a packet arrives on a router, the router uses a routing table to decide the best path for the destination network. The destination may be in the local area network or in another country. The delivery of this packet is the responsibility of the router. The effectiveness of communications between networks depends, on the ability of routers to send packets in the most efficient way possible. The router does not have video and sound adapters like the computer. It has special ports and network interface cards to interconnect devices to other networks. The important parts of the router are the following:
- The central processing unit (CPU)
- Memory and storage (RAM, ROM, NVRAM, Flash, hard drive)
- Operating system (OS)
A router is basically a special computer. It has a CPU to execute operating system instructions, such as system initialization, routing functions, and switching functions. It has also a memory to store data temporarily and permanently. Cisco devices use the Cisco Internetwork Operating System (IOS) as the system software. I already explained the router memory in one of my earlier article :
Routers CPU, OS and Memory
People don’t know about the presence of various routers on their own network or on the Internet. Users want to access web pages, read and send emails, and download music, videos, software without knowing whether the server accessed is on their own network or on another network. Only the networking professionals can understand the routers responsibility for forwarding packets from network to network, from the source to the destination.
A router connects and communicates between multiple networks, it has multiple interfaces that each belong to a different IP network. When the IP packet received to the router on one of the interfaces, it determines the interface for forwarding the packet to the destination. The interface router forwarding the packet may be the destination, or it may be a network linked to another router that is used to reach the destination.
Each network typically required a separate interface. Both local-area networks and wide-area networks interconnect through these interfaces. LANs are containing devices, such as PCs, printers, and servers. WANs connect networks over a large geographical area. For example, a WAN connection is commonly used to connect a LAN to the Internet. The primary functions of the router are the following:
- Select the best path to send packets
- Forward packets toward their destination
The router takes a decision based its routing table to select the best path for forwarding a data. After receiving a packet, the router examines the destination IP address of the packet and searches the best path in the routing table. The routing table knows about the interfaces to send packets for each known network. When a match is found in the routing table, the router sends the packet into the data link frame of the outgoing interface, and the packet is forwarded toward its destination.
The router has different types of interfaces, so it is possible for a router to receive a packet encapsulated in one type of data link frame, and to send the packet out of an interface uses a different type of data link frame. For example, a router receives a packet on an Ethernet interface; but the exit port is Point-to-Point Protocol (PPP), so the data encapsulated in another type of data link frame. There are different data link technologies that a router can connect to include Ethernet; PPP, Frame Relay, DSL, cable, and wireless (802.11, Bluetooth). Routers use both static routes and dynamic routing protocols to learn about remote networks and keep up routing tables.