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1、ThesuccessofTCP/IPasthenetworkprotocoloftheInternetislargelybecauseofitsabilitytoconnecttogethernetworksofdifferentsizesandsystemsofdifferenttypes.Thesenetworksarearbitrarilydefinedintothreemainclasses(alongwithafewothers)thathavepredefinedsizes,eachofwhichcanbedi
2、videdintosmallersubnetworksbysystemadministrators.AsubnetmaskisusedtodivideanIPaddressintotwoparts.Onepartidentifiesthehost(computer),theotherpartidentifiesthenetworktowhichitbelongs.TobetterunderstandhowIPaddressesandsubnetmaskswork,lookatanIP(InternetProtocol)ad
3、dressandseehowitisorganized.IPaddresses:NetworksandhostsAnIPaddressisa32-bitnumberthatuniquelyidentifiesahost(computerorotherdevice,suchasaprinterorrouter)onaTCP/IPnetwork.IPaddressesarcnormallyexpressedindotted-decimalformat,withfournumbersseparatedbyperiods,such
4、as192.168.123.132.Tounderstandhowsubnetmasksareusedtodistinguishbetweenhosts,networks,andsubnetworks,examineanIPaddressinbinarynotation.Forexample,thedotted-decimalIPaddress192.168-123-132is(inbinarynotation)the32bitnumber110000000101000111101110000100.Thisnumberm
5、aybehardtomakesenseof,sodivideitintofourpartsofeightbinarydigits.Theseeightbitsectionsareknownasoctets.TheexampleIPaddress,then,becomes11000000.10101000.01111011.10000100.Thisnumberonlymakesalittlemoresense,soformostuses,convertthebinaryaddressintodotted-decimalfo
6、rmat(192.168.123.132).Thedecimalnumbersseparatedbyperiodsaretheoctetsconvertedfrombinarytodecimalnotation.ForaTCP/IPwideareanetwork(WAN)toworkefficientlyasacollectionofnetworks,theroutersthatpasspacketsofdatabetweennetworksdonotknowtheexactlocationofahostforwhicha
7、packetofinformationisdestined.Routersonlyknowwhatnetworkthehostisamemberofanduseinformationstoredintheirroutetabletodeterminehowtogetthepackettothedestinationhost’snetwork.Afterthepacketisdeliveredtothedestination’snetwork,thepacketisdeliveredtotheappropriatehost.
8、Forthisprocesstowork,anIPaddresshastwoparts.ThefirstpartofanIPaddressisusedasanetworkaddress,thelastpartasahostaddress.Ifyoutaketheexample192.168.123.13
