Routers Study Notes

By BYJU'S Exam Prep

Updated on: September 25th, 2023


A router is a hardware component used to interconnect networks. Routers are devices whose primary purpose is to connect two or more networks and to filter network signals so that only desired information travels between them. Routers are much more powerful than bridges

  • A router has interfaces on multiple networks
  • Networks can use different technologies
  • A router forwards packets between networks
  • Transforms packets as necessary to meet standards for each network
  • Routers are distinguished by the functions they perform:
    • Internal routers: Only route packets within one area.
    • Area border routers: Connect to areas together
    • Backbone routers: Reside only in the backbone area
    • AS boundary routers: Routers that connect to a router outside the AS.
Routers can filter traffic so that only authorized personnel can enter restricted areas. They can permit or deny network communications with a particular Web site. They can recommend the best route for information to travel. As network traffic changes during the day, routers can redirect information to take less congested routes.
  • Routers operate primarily by examining incoming data for its network routing and transport information.
  • Based on complex, internal tables of network information that it compiles, a router then determines whether or not it knows how to forward the data packet towards its destination.
  • Routers can be programmed to prevent information from being sent to or received from certain networks or computers based on all or part of their network routing addresses.
  • Routers also determine some possible routes to the destination network and then choose the one that promises to be the fastest.

Two key router functions of Router:

  • Run routing algorithms/protocol (RIP, OSPF, BGP)
  • Forwarding datagrams from incoming to outgoing link.

Address Resolution Protocol (ARP)

ARP is used to find the physical address of the node when its Internet address is known. Anytime, a host or a router needs to find the physical address of another has on its network; it formats an ARP query packet that includes that IP address and broadcasts it over the network. Every host on the network receives and processes the ARP packet, but the intended recipient recognizes its Internet address and sends back its physical address.


Reverse Address Resolution Protocol (RARP)

This protocol allows a host to discover its Internet address when it has known only its physical address. RARP works much like ARP. The host wishing to retrieve its Internet address broadcasts a RARP query packet that contains its physical address to every host of its physical network. A server on the network recognizes the RARP packet and return the hosts Internet address.


Internet Control Massage Protocol (ICMP)

The ICMP is a mechanism used by hosts and routers to send notifications of datagram problems back to the sender. IP is essentially an unreliable and connectionless protocol. ICMP allows IP (Internet Protocol) to inform a sender if a datagram is un-deliverable.
ICMP uses each test/reply to test whether a destination is reachable and responding. It also handles both control and error messages but its sole function is to report problems not correct them.


Internet Group Management Protocol(IGMP)

The IP can be involved in two types of communication uni-tasking and multitasking. The IGMP protocol has been designed to help a multitasking router to identify the hosts in a LAN that are members of a multicast group.


Addressing at Network Layer

In addition to the physical addresses that identify individual devices, the Internet requires an additional addressing connection to an address that identifies the connection of a host of its network. Every host and router on the Internet has an IP address which encodes its network number and host number. The combination is unique in principle; no 2 machines on the Internet have the same IP address.



A firewall is a device that prevents unauthorized electronic access to your entire network.
The term firewall is generic and includes many different kinds of protective hardware and software devices. Routers comprise one kind of firewall.
Most firewalls operate by examining incoming or outgoing packets for information at OSI level 3, the network addressing level.
Firewalls can be divided into 3 general categories: packet-screening firewalls, proxy servers (or application-level gateways), and stateful inspection proxies.
  • Packet-screening firewalls examine incoming and outgoing packets for their network address information. You can use packet-screening firewalls to restrict access to specific Web sites or to permit access to your network only from specific Internet sites.
  • Proxy servers (also called application-level gateways) operate by examining incoming or outgoing packets not only for their source or destination addresses but also for information carried within the data area (as opposed to the address area) of each network packet. The data area contains information written by the application program that created the packet—for example, your Web browser, FTP, or TELNET program. Because the proxy server knows how to examine this application-specific portion of the packet, you can permit or restrict the behaviour of individual programs.
  • The Stateful inspection proxies monitor network signals to ensure that they are part of a legitimate ongoing conversation (rather than malicious insertions)

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