I'veredrawnthisscenariodownhere, um, in a littlebitmoredetail.
Sowe'vegotherEthernetnetworkoverhereontheleft, andthisisthisis a multipointlinkhere.
So I'veshownallthecomputersconnected, youknow, tothisverticallinejusttokindofshowthatanyofthemcansendframestoanyoftheotheronesthatairthatareconnectedtothisnetwork.
SoifHostbehassomesortofuniversaladdressthatit's understoodeverywhere, thenallofthesedevicesinthemiddleofthesetheseroutersinthemiddlehere I wouldbeabletousethataddresstoforwarddatatotherightplace.
Andsotheaddressthatweuseiscalledan I P addressorInternetProtocoladdress.
Internetprotocolfor I P addressandthewordInternetliterallymeansenterorbetweennetworks.
AndsotheInternetProtocoland I P addressesareexactlywhatwewouldusetosendinformationbetweennetworks, whichiswhatwe'retryingtodofromthisnetworktothisnetwork.
Andsowhatthese I P routersdoisthey'reconnectedtomultiplenetworksandthere's someframesarrivinghere.
Let's sayoninterfaceoneofthisSanFranciscorouterandinsidethoseframesis A is a packetofdatawith a destination I p address, andsotheroutersjobistolookatthatdestinationaddressanddecidewhetherthepackageshouldbeforwardedoutthisinterfaceorthisinterface.
Andsoifitdecidestoforwardthepacketoutinterfacethree, let's saythenitwillencapsulatethat I p packetinside a PPPframethenandsenditontothenextrouteroverhere.
AndthentheDenverrouterwilldothesamething.
It'lllookatthepacketinsidethePPPframe, lookatthedestination i p addressanddecidewhichoftheseinterfacesit's gonnaforwarditoutof.
Sothat's whatwritersdo.
Theyreceivepacketsoneachother, interfaceson, make a decisionaboutwhichotherinterfacetoforwardthemoutof.
Andsothat's theprocessofforwarding.
Andsoforthat, towork, eachrouterneedstoehavewhat's called a forwardingtablethattellsithowtogettoeachdestination.
So, forexample, thisrouterinSanFrancisconeedstosomehowlearnthattogettothe I p addressforthishostoverhereinNewYork, itshouldforwardthepacketsoutinterfacethreeandnotinterfacetoorsomewhereelse.
Soyoumightimaginethateachoftheseroutershave a forwardingtablewithuptofourbillionentries, oneforeach i p address, saying, Youknow, togettothisaddress, gooutthisinterfaceorgooutthatinterface, andinprinciplewecoulddothat.