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1、ThesuccessofTCP/IPasthenetworkprotocoloftheInternetislargelybecauseofitsabilitytoconnecttogethernetworksofdifferentsizesandsystemsofdifferenttypes.Thesenetworksarearbitrarilydefinedintothreemainclasses(alongwithafewothers)thathavepredefinedsizes,eachofwhichcanbedividedinto
2、smallersubnetworksbysystemadministrators.AsubnetmaskisusedtodivideanIPaddressintotwoparts.Onepartidentifiesthehost(computer),theotherpartidentifiesthenetworktowhichitbelongs.TobetterunderstandhowIPaddressesandsubnetmaskswork,lookatanIP(InternetProtocol)addressandseehowitis
3、organized.IPaddresses:NetworksandhostsAnIPaddressisa32-bitnumberthatuniquelyidentifiesahost(computerorotherdevice,suchasaprinterorrouter)onaTCP/IPnetwork.IPaddressesarenormallyexpressedindotted-decimalformat,withfournumbersseparatedbyperiods,suchas192.168.123.132.Tounderst
4、andhowsubnetmasksareusedtodistinguishbetweenhosts,networks,andsubnetworks,examineanIPaddressinbinarynotation.Forexample,thedotted-decimalIPaddress192.168.123.132is(inbinarynotation)the32bitnumber110000000101000111101110000100.Thisnumbermaybehardtomakesenseof,sodivideitinto
5、fourpartsofeightbinarydigits.Theseeightbitsectionsareknownasoctets.TheexampleIPaddress,then,becomes11000000.10101000.01111011.10000100.Thisnumberonlymakesalittlemoresense,soformostuses,convertthebinaryaddressintodotted-decimalformat(192.168.123.132).Thedecimalnumberssepara
6、tedbyperiodsaretheoctetsconvertedfrombinarytodecimalnotation.ForaTCP/IPwideareanetwork(WAN)toworkefficientlyasacollectionofnetworks,theroutersthatpasspacketsofdatabetweennetworksdonotknowtheexactlocationofahostforwhichapacketofinformationisdestined.Routersonlyknowwhatnetwo
7、rkthehostisamemberofanduseinformationstoredintheirroutetabletodeterminehowtogetthepackettothedestinationhost'snetwork.Afterthepacketisdeliveredtothedestination'snetwork,thepacketisdeliveredtotheappropriatehost.Forthisprocesstowork,anIPaddresshastwoparts.ThefirstpartofanIPa
8、ddressisusedasanetworkaddress,thelastpartasahostaddress.Ifyoutaketheexample192.168.123.13
