THE INTERNET ROUTING HIERARCHY

SO HOW DO PACKETS FIND THEIR ACROSS THE INTERNET? DOES EVERY COMPUTER CONNECTED TO THE INTERNET KNOW WHERE THE OTHER COMPUTER ARE? DO PACKETS SIMPLY  GET "BROADCAST" TO EVERY COMPUTER ON THE INTERNET ?THE ANSWER TO BOTH THE PRECEEDING QUESTIONS IS 'NO" . NO COMPUTER KNOWS WHERE ANY OF THE OTHER COMPUTERS ARE, AND PACKETS DO NOT GET SENT TO EVERY COMPUTER.THE INFORMATION USED TO GET INTERNET TO THEIR DESTINATIONS ARE CONTAINED IN ROUTING TABLES KEPT BY EACH ROUTER CONNECTED TO THE INTERNET
      ROUTER ARE PACKET SWITCHES
A ROUTER IS USUALLY CONNECTED  BETWEEN NETWORKS TO ROUTE PACKETS BETWEEN THEM.EACH ROUTER KNOWS ABOUT IT's SUN-NETWORKS AND WHICH IP ADDRESSES THEY USE.THE ROUTE USUALLY DOESN'T  KNOW WHAT IP ADDRESSES ARE "ABOVE" IT.EXAMINE DIAGRAM BELOW .THE BLACK BOXES CONNECTING THE BACKBONE ARE ROUTERS.THE LARGE NSP BACKBONE AT THE TOP ARE CONNECTED AT A NAP. UNDER THEM ARE SEVERAL SUN-NETWORKS ,AND UNDER THEM, MORE SUB-NETWORKS. AT THE BOTTOM ARE TWO LOCAL AREA NETWORKS WITH COMPUTERS ATTACHED.

 

WHEN A PACKET ARRIVES AT A ROUTER ,THE ROUTER EXAMINES THE IP ADDRESS PUT THERE BY THE IP PROTOCOL LAYER ON THE ORIGINATING COMPUTER.THE ROUTER CHECKS IT's  ROUTING TABLES .IF THE NETWORKS CONTAINING THE IP ADDRESS IN NOT FOUND , THEN THE ROUTER SENDS THE PACKET ON A DEFAULT ROUTE,USUALLY UP THE BACKBONE HIERARCHY TO THE NEXT ROUTER. HOPEFULLY THE NEXT ROUTE WILL KNOW WHERE TO SEND THE PACKET.IF IT DOES NOT ,AGAIN  THE PACKET IS ROUTED UPWARDS UNTIL IT REACHES A NSP BACKBONE . THE ROUTERS CONNECTED TO THE  NSP BACKBONE HOLD THE LARGEST ROUTING TABLES AND HERE THE PACKET SMALLER AND SMALLER NETWORKS UNTIL IT FIND IT'S DESTINATION. 

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