livellorete indizzamentoip (v4) e...

Reti(già “Reti di Calcolatori”)

Livello ReteIndizzamento IP (v4) e inoltro dei pacchetti

RenatoLoCigno

http://disi.unitn.it/locigno/teaching-duties/reti

Acknowledgement

• Credits– Partof thematerialis based onslides provided by thefollowing authors

• JimKurose,KeithRoss,“ComputerNetworking:ATopDownApproach,”4thedition,Addison-Wesley,July2007

• DouglasComer,“ComputerNetworksandInternets,”5thedition,PrenticeHall

• Behrouz A.Forouzan,Sophia Chung Fegan,“TCP/IPProtocolSuite,”McGraw-Hill,January 2005

• Latraduzione,sepresente,èingeneraleopera(eresponsabilità)deldocente

[email protected] LivelloRete- IPv4 2

Contenuto e temi

• Spaziodiindirizzamento

• IndirizziIPelorouso

• Consegnadeipacchetti

• ConfigurazionedeiPCedellereti

• InstradamentoeRouting


1- Fisico

Livello Rete

Livellidiapplicazione(utente)

Livellidirete2- Collegamentodati

Fisico

3- Rete

4- Trasporto

5- Sessione

6- Presentazione

7- Applicazione

Collegamentodati:Ethernet,PPP,ATM,...

Rete:IP

Trasporto:TCP- UDP

Applicaz.:HTTP,E-mail



Consegna Diretta e Indiretta

NtwA165.5.1.0/24

NtwB98.23.0.0/16

NtwC201.12.82.0/24

HostA

HostC

165.5.1.5

201.12.82.4

NtwD201.12.83.0/24

NtwE198.35.112.0/24

Questo router è non è collegato allarete C, come fa a conoscere l’host201.12.82.4? E come fa a inoltrare i pacchetticorrettamente verso ques’ultimo?

Prospettiva globale

• Trasportodeipacchettidasorgentearicevitore.Ipacchetticontengonounsegmentodilivellotrasporto

• IpacchettisonoincapsulatiintrameL2

• Alricevitoreisegmentisonoestrattidaipacchettieconsegnatiallivellotrasporto

• Iprotocollidiretesonointuttiglihost erouter

• Unrouterdeveesaminarel’intestazionedituttiipacchetticheloattraversano

Applicaz.

Trasporto

Rete

DataLink

Fisico

Applicaz.

Trasporto

Rete

DataLink

Fisico

Rete

L2

L1

L2

L1

Rete

L2

L1

L2

L1

IPL2

IP

IPL2

IPL2

IP

IPL2

IPL2

IPL2IPL2

IP

IPL2

IPL2

IP


Funzioni fondamentali del livello rete

• Instradamento(Routing)– Trovareilpercorsodallasorgentealladestinazioneè AlgoritmidiRouting

• Simileapianificareunviaggio:devodeterminarelestradedafareegliincrociincuicambiarelamiastrada

• Inoltro(Forwarding)– Funzionecheesegueiltrasportodeipacchettidagliingressialleuscitedeirouter...datocheilpercorsoègiànoto

• Simileaprenderel’uscitagiustadiunarotonda,sapendochedevoandareinunaspecificadirezione

• Entramberichiedonounospaziodiindirizzamentoappropriato...eirelativiindirizzi


1

23

0111

valore dell’indirizzonel pacchetto in arrivo

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Relazione tra routing e forwarding


Schema di architettura dei Router

• duefunzionifondamentali:– eseguireiprotocolliealgoritmidiinstradamento(RIP,OSPF,BGP)– inoltrareidatagrammi(pacchetti)dagliingressialleuscite


IL PROTOCOLLO IP(VERSIONE 4)


Il Datagramma IP

• TCP/IPusailtermine“IPdatagram”peridentificareunpacchettodilivellorete

• Ciascundatagrammaècompostodaunaintestazione (header)– da20a60bytes checontengonoleinformazioniessenzialiperl’instradamentoelaconsegna

• seguitadaidatitrasportati (payload)– Ladimensionedeipayload nonèfissa– Ladimensioneeffettivaèdeterminatadall’applicazionee/odalprotocolloditrasporto

– C’e` unadimensionemassimadi64kB(65536)


Formato del Datagramma IP

• L’headercontieneinformazioniutiliperl’instradamentodeldatagramma:– L’indirizzodellasorgente(chihainviatoperprimoildatagramma)

– L’indirizzodelladestinazione(chiriceveràildatagrammasetuttovabene)

– Iltipodiprotocollochehageneratoidati– ...

• GliindirizzisonoditipoIP(ovvio)• GliindirizziMACsono“esterni”aldatagramma• Icampisonodidimensionefissaperefficienzadimanipolazione


The IP Datagram Header Format

[email protected] LivelloRete- IPv4


• VERS:Eachdatagrambeginswitha4-bitprotocolversionnumber• H.LEN:4-bitheaderspecifiesthenumberof32-bitquantitiesinthe

header– Ifnooptionsarepresent,thevalueis5

• TypeofService(ToS)– 8-bitfieldthatcarriesaclassofserviceforthedatagram

• potentiallyusedforDiffServ andECN(ExplicitCongestionNotification)

• seldomusedinpractice• TOTALLENGTH:16-bitintegerthatspecifiesthetotalnumberof

bytesincludingboththeheaderandthedata



• IDENTIFICATION– 16-bitnumber(usuallysequential)assignedtothedatagram

• usedtogatherallfragmentsforreassemblytothedatagram• FLAGS

– 3-bitfieldwithindividualbitsspecifyingwhetherthedatagramisafragment

• Ifso,thenwhetherthefragmentcorrespondstothelastpieceoftheoriginaldatagram

• FRAGMENTOFFSET– 13-bitfieldthatspecifieswhereintheoriginaldatagramthedatainthisfragmentbelongs

– thevalueofthefieldismultipliedby8toobtainanoffset



• TIMETOLIVE(TTL)– 8-bitintegerinitializedbytheoriginalsender– itisdecrementedbyeachrouter thatprocessesthedatagram– ifthevaluereacheszero(0)thedatagramisdiscarded andanerrormessageissentbacktothesource

• PROTOCOL– 8-bitfieldthatspecifiesthetypeofthepayload,i.e.,theprotocolabove(e.g.,6forTCP,17forUDP)

• HEADERCHECKSUM– 16-bitones-complementchecksumofheaderfields

• SOURCEIPADDRESS– 32-bitInternetaddressoftheoriginalsender



• DESTINATIONIPADDRESS– The32-bitInternetaddressoftheultimatedestination

• IPOPTIONS– Optionalheaderfieldsusedtocontrolroutinganddatagramprocessing

– Mostdatagramsdonotcontainanyoptions• PADDING

– Ifoptionsdonotendona32-bitboundary• zerobitsofpaddingareaddedtomaketheheaderamultipleof32bits


FRAMMENTAZIONE DEI PACCHETTI IP

L’USO DELLA FRAMMENTAZIONE È “DEPRECATO”. MOLTI ROUTER SEMPLICEMENTE NON LA IMPLEMENTANOE SCARTANO IL PACCHETTO.


MTU and Datagram Fragmentation

• Eachhardwaretechnologyspecifiesthemaximumamountofdatathataframecancarry– ThelimitisknownasaMaximumTransmissionUnit (MTU)

• NetworkhardwareisnotdesignedtoacceptortransferframesthatcarrymoredatathantheMTUallows– AdatagrammustbesmallerorequaltothenetworkMTU

• oritcannotbeencapsulatedfortransmission• Inaninternetthatcontainsheterogeneousnetworks,MTU

restrictionscreateaproblem• AroutercanconnectnetworkswithdifferentMTUvalues

– adatagramthatarouterreceivesoveronenetworkcanbetoolargetosendoveranothernetwork


MTUs for some networks


variabile


• Example:arouterinterconnectstwonetworkswithMTUvaluesof1500and1000– HostH1 attachestoanetworkwithanMTUof1500

• andcansendadatagramthatisupto1500octets– HostH2 attachestoanetworkthathasanMTUof1000

• whichmeansthatitcannotsend/receiveadatagramlargerthan1000octets

– IfhostH1 sendsa1500-octetdatagramtohostH2

• routerRwillnotbeabletoencapsulateitfortransmissionacrossnetwork2



• WhenadatagramislargerthantheMTUofthenetworkoverwhichitmustbesent– therouterdividesthedatagramintosmallerpiecescalledfragments

– andsendseachfragmentindependently• Afragmenthasthesameformatasotherdatagrams

– abitintheFLAGSfieldoftheheaderindicateswhetheradatagramisafragmentoracompletedatagram

• Otherfieldsintheheaderareassignedinformationfortheultimatedestinationtoreassemblefragments– toreproducetheoriginaldatagram

• TheFRAGMENTOFFSETspecifieswhereintheoriginaldatagramthefragmentbelongs



• ArouterusesthenetworkMTUandtheheadersizetocalculate– themaximumamountofdatathatcanbesentineachfragment– andthenumberoffragmentsthatwillbeneeded

• Theroutercreatesthefragments– Itusesfieldsfromtheoriginalheadertocreateafragmentheader

– Itcopiestheappropriatedatafromtheoriginaldatagramintothefragment

– Transmitstheresult


Flags field


Reassembling a Datagram

• Example:packetssentfromH1toH2– ifhostH1sendsa1500-octetdatagramtohostH2,routerR1willdividethedatagramintotwofragments,whichitwillforwardtoR2

– RouterR2doesnotreassemblethefragments• InsteadRusesthedestinationaddressinafragmenttoforwardthefragmentasusual

– Theultimatedestinationhost,H2,collectsthefragmentsandreassemblesthemtoproducetheoriginaldatagram


Reassembling a Datagram

• Requiringtheultimatedestinationtoreassemblefragmentshastwoadvantages:– Itreducestheamountofstateinformationinrouters

• Whenforwardingadatagram,arouterdoesnotneedtoknowwhetherthedatagramisafragment

– Itallowsroutes tochangedynamically• Ifanintermediaterouterweretoreassemblefragments,allfragmentswouldneedtoreachtherouter

• Bypostponingreassemblyuntiltheultimatedestination– IPisfreetopasssomefragmentsfromadatagramalongdifferentroutesthanotherfragments


The Consequence of Fragment Loss

• Adatagramcannotbereassembleduntilallfragmentsarrive• Thereceivermustsave(buffer)thefragments

– Incasemissingfragmentsareonlydelayed– Areceivercannotholdfragmentsanarbitrarilylongtime

• IPspecifiesamaximumtimetoholdfragments• Whenthefirstfragmentarrivesfromagivendatagram

– thereceiverstartsareassemblytimer• Ifallfragmentsofadatagramarrivebeforethetimerexpires

– thereceivercancelsthetimerandreassemblesthedatagram• Otherwisethereceiverdiscardsthefragments


GLI INDIRIZZI DI INTERNET:IPV4


Gli indirizzi di Internet

• Lospaziodiindirizzamentoèuncomponentecritico• Tuttiglihost eirouterdevonousareunoschemadiindirizzamento

uniforme• GliindirizziUnicast,cheidentificanounaspecificainterfacciadevono

essereunici• EsistonoduespazidiindirizzamentospecificatiperInternet

– IPv4:quelloattualmenteinusoconindirizzia32bit– IPv6:ilsistemadiindirizzamentocheavrebbedovutosostituireIPv4,machecontinuaanonfarlo

• indirizzia128bit• funzioni“avanzate”• esistonomolte“isole”IPv6eormaituttiirouterdeimaggiorivendor losupportano


The IP Addressing Scheme

• IPaddressesaresuppliedbyprotocolsoftware

• Eachnetworkinterfaceisassignedaunique32-bitnumber– TheinterfaceIPaddress orInternetaddress

• WhensendingapacketacrosstheInternet,sender’sprotocolsoftwaremustspecify– itsown32-bitIPaddress(thesourceaddress)– andtheaddressoftheintendedrecipient(thedestinationaddress)

• Routersonlyusethedestinationaddressforforwrding androuting


Dotted Decimal Notation

• Insteadofwriting32bits,anotationmoreconvenientforhumanstounderstandisused,knownasdotteddecimalnotation– expresseach8-bitsectionofa32-bitnumberasadecimalvalue– useperiodstoseparatethesections

• Dotteddecimaltreatseachoctet(byte)asanunsignedbinaryinteger– thesmallestvalue,0

• occurswhenallbitsofanoctetarezero(0)– thelargestvalue,255

• occurswhenallbitsofanoctetareone(1)– dotteddecimaladdressesrange

0.0.0.0through255.255.255.255


Dotted Decimal Notation: examples


The IP Address Hierarchy

• IPaddressisdividedintotwoparts:• Aprefixè identifiesthephysicalnetworktowhichthehostis

attached(alsoknownadNetID)– EachnetworkintheInternetisassignedauniquenetworknumber

• Asuffixè identifiesaspecificinterfaceonthenetwork(alsoknownadHostID)– EachNIConagivennetworkisassignedauniquesuffix

• IPaddressschemeguaranteestwoproperties:– Eachcomputerisassignedauniqueaddress– Networknumbers(prefix)mustbecoordinatedglobally– Suffixesareassignedlocallywithoutglobalcoordination


INDIRIZZAMENTO CON CLASSI (OBSOLETO)

Schemadiorganizzazionedegliindirizziusatofinoallametàdeglianni‘90ebasatosuunadivisionestaticatraNetID eHostID

Èusoancoraoggiriferirel’organizzazionedegliindirizziadunconcetto(eterminologia)diclasse


Original Classes of IP Addresses

• Internetcontainsafewlargephysicalnetworksandmanysmallnetworks– thedesignerschoseanaddressingschemetoaccommodateacombinationoflargeandsmallnetworks

• Theoriginalclassful IPaddressingdividedtheIPaddressspaceinto3primaryclasses– eachclasshasadifferentsizeprefixandsuffix

• ThefirstfourbitsofanIPaddressdeterminedtheclasstowhichtheaddressbelonged– Itspecifieshowtheremainderoftheaddresswasdividedintoprefixandsuffix


Original Classes of IP Addresses


Division of the Address Space

• Theclassful schemedividedtheaddressspaceintounequalsizes• Thedesignerschoseanunequaldivisiontoaccommodateavariety

ofscenarios– Forexample,althoughitislimitedto128networks,classAcontainshalfofalladdresses

• ThemotivationwastoallowmajorISPstoeachdeployalargenetworkthatconnectedmillionsofcomputers

• ButAclasseswereassignedtosmallnetworksallintheUS...– Similarly,themotivationforclassCwastoallowanorganizationtohaveafewcomputersconnectedonaLAN


Division of the Address Space


Authority for Addresses

• InternetCorporationforAssignedNamesandNumbers(ICANN)authorityhasbeenestablished– tohandleaddressassignmentandadjudicatedisputes

• ICANNdoesnotassignindividualprefixes– Instead,ICANNauthorizesasetofregistrars todoso

• RegistrarsmakeblocksofaddressesavailabletoISPs– ISPsprovideaddressestosubscribers

• Toobtainaprefix– acorporationusuallycontactsanISP


INDIRIZZAMENTO SENZA CLASSI E CIDR

SchemainusoattualecondivisionedinamicatraNetID eHostID

CIDR(Classless Inter-DomainRouting)consentel’instradamentoglobalesenzausarelanozionediclasse


Subnets and Classless Addressing

• AstheInternetgrewtheoriginalclassfuladdressingschemebecamealimitation

• EveryonedemandedaclassAorclassBaddress– Sotheywouldhaveenoughaddressesforfuturegrowth

• butmanyaddressesinclassAandBwereunused• Twomechanisms,closelyrelated,weredesignedtoovercomethe

limitation– Subnetaddressing– Classlessaddressing

• Insteadofhavingthreedistinctaddressclasses,allowthedivisionbetweenprefix/suffixonanarbitrarybitboundary


Classless Addressing: Motivation

• ConsideranISPthathandsoutprefixes.SupposeacustomeroftheISPrequestsaprefixforanetworkthatcontains55hosts– classfuladdressingrequiresacompleteclassCprefix– only6bitsofsuffixareneededtorepresentallpossiblehostvalues

• means190ofthe254possiblesuffixeswouldneverbeassigned

– mostoftheclassCaddressspaceiswasted• Fortheaboveexample

– classlessaddressingallowstheISPtoassign• aprefixthatis26bitslong• asuffixthatis6bitslong


Classless Addressing: Example

• AssumeanISPownsaclassCprefix– Classfuladdressingassignstheentireprefixtooneorganization

• Withclasslessaddressing– theISPcandividetheprefixintoseverallongerprefixes

• Forinstance,theISPcandivideaclassCprefixinto4longerprefixes– eachonecanaccommodateanetworkofupto62hosts

• all0sandall1sarereserved• TheoriginalclassCaddresshas8bitsofsuffix

– andeachoftheclasslessaddresseshas6bitsofsuffix• Thus,insteadofwastingaddresses

– ISPcanassigneachofthe4classlessprefixestoasubscriber


Classless Addressing: Example


Address (subnet) Masks

• HowcananIPaddressbedividedatanarbitraryboundary?• Theclasslessandsubnetaddressingschemesrequirehostsand

routerstostoreanadditionalpieceofinformation:– avaluethatspecifiestheexactboundarybetweenprefixandsuffix

• Tomarktheboundary,IPusesa32-bitvalue– knownasanaddressmask,alsocalledasubnetmask

• Whystoretheboundarysizeasabitmask?– Amaskmakesprocessingefficient

• Hostsandroutersneedtocomparethenetworkprefixportionoftheaddresstoavalueintheirforwardingtables– Thebit-maskrepresentationmakesthecomparisonefficient


Address Masks

• Supposearouterisgiven– adestinationaddress,D– anetworkprefixrepresentedasa32-bitvalue,N– a32-bitaddressmask,M

• AssumethetopbitsofNcontainanetworkprefix,andtheremainingbitshavebeensettozero

• Totestwhetherthedestinationliesonthespecifiednetwork,therouterteststhecondition:

N==(D&M)• Therouter

– usesthemaskwitha“logicaland(&)”operationtosetthehostbitsofaddressDtozero(0)

– andthencomparestheresultwiththenetworkprefixN


Address Masks: Example

• Considerthefollowing32-bitnetworkprefix:10000000000010100000000000000000 à 128.10.0.0

• Considera32-bitmask:11111111111111110000000000000000 à 255.255.0.0

• Considera32-bitdestinationaddress,whichhasa10000000000010100000001000000011 à 128.10.2.3

• Alogical &betweenthedestinationaddressandtheaddressmaskextractsthehigh-order16-bits10000000000010100000000000000000 à 128.10.0.0


CIDR Notation

• ClasslessInter-DomainRouting(CIDR)• Considera maskdefiningasubnetwith26 nodes

– Ithas26bitsof1sfollowedby6bitsof0s– Indotteddecimal,themaskis:255.255.255.192

• ThegeneralformofCIDRnotationis:ddd.ddd.ddd.ddd/m– ddd isthedecimalvalueforanoctetoftheaddress– m isthenumberofonebitsinthemask

• Thus,onemightwritethefollowing:192.5.48.69/26– whichspecifiesamaskof26bits


A CIDR Example

• AssumeanISPhasthefollowingblock128.211.0.0/16• SupposetheISPhas2customers

– onecustomerneeds12IPaddressesandtheotherneeds9• TheISPcanassign

– customer1CIDR:128.211.0.16/28– customer2CIDR:128.211.0.32/28– bothcustomershavethesamemasksize(28bits),theprefixesdiffer


A CIDR Example (cont’d)

• Thebinaryvalueassignedtocustomer1is:– 10000000110100110000000000010000

• Thebinaryvalueassignedtocustomer2is:– 10000000110100110000000000100000

• Thereisnoambiguity– Eachcustomerhasauniqueprefix– Moreimportant,theISPretainsmostoftheoriginaladdressblock

• itcanthenallocatetoothercustomers


CIDR Host Addresses

• OnceanISPassignsacustomeraCIDRprefix– thecustomercanassignhostaddressesforitsnetworkusers

• Supposeanorganizationisassigned128.211.0.16/28– theorganizationwillhave4-bitstouseasahostaddressfield

• Disadvantageofclasslessaddressing– Becausethehostsuffixcanstartonanarbitraryboundary,valuesarenoteasytoreadindotteddecimal


CIDR Host Addresses


INDIRIZZI PRIVATI, SPECIALI E INDIRIZZI DEI ROUTERNontuttigliindirizziIPsonoutilizzabili,alcuniindirizzihannosignificatosolointernoalcomputerealtriconsentonodifareilbootstrap dellemacchineprimacheabbianounindirizzoIPconcuicomunicare.IRoutersonomacchineconpiùindirizziIP...anchesenonsempreconpiùinterfaccefisichedicomunicazione.


Indirizzi pubblici e privati

• NontuttigliindirizziIPUnicast validisonouguali• Alcuniindirizzisonostatidefiniti“privati”enonsonoinstradabiliin

Internet– PossonoessereusatipercostruireIntra-netprivate

• Unhost conindirizzoIPprivatohabisognodiunaapparatoattivochetraducaopportunamenteisuoipacchettiperaccedereaInternet

• NAT:NetworkAddress Translator– Mappala5-tuplacheidentificaunflussosuun’altra5-tuplaconindirizzopubblico,lavoraalivelloL3/L4

• Proxy– GatewaydiL7,cheinterconnettealivellodisingolaapplicazione


Indirizzi Privati

• 10.0.0.0– 10.255.255.255• 172.16.0.0– 172.31.255.255• 192.168.0.0– 192.168.255.255

• Unindirizzoprivatopuòessereriutilizzatoinmoltipostidiversi• All’internodiunastessaretedevonoessereuniciepossonoessere

“routati”finoaun“border router”cheinveceimpediscediandareversoInternet

• NormalmentesonoassegnatitramiteDHCPenonsonoassegnatistaticamenteaunhost

• Nonc’è una reale differenza tra i tre gruppi diindirizzi,maingenere i router“domestici”usano 192.168.x.y/24


Special IP Addresses

• IPdefinesasetofspecialaddressformsthatarereserved– Thatis,specialaddressesareneverassignedtohosts

• Examples:– NetworkAddress– DirectedBroadcastAddress– LimitedBroadcastAddress– ThisComputerAddress– LoopbackAddress– Multicastaddresses


Network Address

• Itisconvenienttohaveanaddressthatcanbeusedtodenotetheprefix assignedtoagivennetwork

• IPreserveshostaddresszero– andusesittodenoteanetwork

• Thus,theaddress128.211.0.16/28denotesanetwork– becausethebitsbeyondthe28arezero– 10000000110100110000000000010000

• Anetworkaddressshouldneverappearasthedestinationaddressinapacket


Directed Broadcast Address

• Tosimplifybroadcasting(sendtoall)– IPdefinesadirectedbroadcastaddressforeachphysicalnetwork

• Whenapacketissenttoanetwork'sdirectedbroadcast– asinglecopyofthepackettravelsacrosstheInternet

• untilitreachesthespecifiednetwork– thepacketisthendeliveredtoallhostsonthenetwork

• Thedirectedbroadcastaddressforanetworkisformedbyaddingasuffixthatconsistsofall1bitstothenetworkprefix– 10000000110100110000000000011111


Limited Broadcast Address

• Limitedbroadcastreferstoabroadcastonadirectly-connectednetwork:– informally,wesaythatthebroadcastislimitedtoa“singleLAN”meaningthatitwillneverbeforwardedbyarouter,evenifthe“LAN”canbeahugeCampusLANwithhundredsofcomputers

• Limitedbroadcastisusedduringsystemstartup– byacomputerthatdoesnotyetknowthenetworknumber

• IPreservestheaddressconsistingof32-bitsof1s– 11111111111111111111111111111111

• Thus,IPwillbroadcastanypacketsenttotheall-1saddressacrossthelocalnetwork


This Computer Address

• AcomputerneedstoknowitsIPaddress– beforeitcansendorreceiveInternetpackets

• TCP/IPcontainsprotocolsacomputercanusetoobtainitsIPaddressautomaticallywhenthecomputerboots– …butthestartupprotocolsalsouseanIPtocommunicate

• Whenusingsuchstartupprotocols– acomputercannotsupplyacorrectIPsourceaddress– TohandlesuchcasesIPreservestheaddressthatconsistsofall0stomeanthiscomputer

– 00000000000000000000000000000000


Loopback Address

• Loopbackaddressusedtotestnetworkapplications– e.g.,forpreliminarydebuggingafteranetworkapplicationhasbeencreated

• Aprogrammermusthavetwoapplicationprogramsthatareintendedtocommunicateacrossanetwork

• Insteadofexecutingeachprogramonaseparatecomputer– theprogrammerrunsbothprogramsonasinglecomputer– andinstructsthemtousealoopbackaddresswhencommunicating

• Whenoneapplicationsendsdatatoanother– datatravelsdowntheprotocolstacktotheIPsoftware– thenforwardsitbackupthroughtheprotocolstacktothesecondprogram


Loopback Address (cont’d)

• IPreservesthenetworkprefix127/8 forusewithloopback• Thehostaddressusedwith127isirrelevant

– allhostaddressesaretreatedthesame– programmersoftenusehostnumber1– soitmakes127.0.0.1 themostpopularloopbackaddress

• Duringloopbacktestingnopacketseverleaveacomputer– theIPsoftwareforwardspacketsfromoneapplicationtoanother

• Theloopbackaddressneverappearsinapackettravelingacrossanetwork


Summary of Special IP Addresses


Indirizzo Multicast

• Internetammettel’inviodipacchettia“molti”• Iroutersipreoccupano(complesso,nonlovediamoinquesto

corso)dicapireilpuntoottimodoveduplicarel’informazione• Unpacchettomulticast è inviato aunindirizzo di“gruppo”• InIPv4:ClassD,iniziano per1110• 224.0.0.0– 239.255.255.255• Esistonogruppimulticast “well known”• 224.0.0.1:All Hosts onthis Subnet• 224.0.0.2:All Routers onthis Subnet• Glialtripossonoessereusatiperapplicazioniproprietarieonuove• PurtropponontuttigliISPpermettonotrafficomulticast senonper

lagestionedeiprotocollidirouting stessi


Multicast Routing

• Multicast:deliveryofsamepackettoagroupofreceiverswiththeminimumoverhead

• Multipleunicastvs.multicast– Hostbasedvs.networkbased


Routers and IP Addresses

• EachrouterisassignedtwoormoreIPaddresses– oneaddressforeach logicalnetworktowhichtherouterattaches

• Tounderstandwhy,recalltwofacts:– Arouter connectsmultipleIPnetworks(bydefinition)– EachIPaddresscontainsaprefixthatspecifiesa logicalnetwork

• AsingleIPaddressdoesnotsufficeforarouter– becauseeachrouterconnectstomultiplenetworks– andeachnetworkhasauniqueprefix

• TheIPschemecanbeexplainedbyaprinciple:– AnIPaddressdoesnotidentifyaspecificcomputer– eachaddressidentifies ininterface,i.e.,alogicalconnectionbetween

acomputerandanetwork– Acomputerwithmultiplenetworkconnections(e.g.,arouter)must

beassignedoneIPaddressforeachconnection


An Example


Un altro esempio


Campus LAN con 4 reti IP private: 192.168.1.0/24192.168.2.0/24192.168.3.0/24192.168.4.0/24

Routerdicampuscheinterconnettele4 retiedinoltraalRouterNATdicollegamentoversoInternet:5 indirizziIPtuttisullastessainterfacciafisicadellacampusLAN

192.168.1.1192.168.2.1192.168.3.1192.168.4.1192.168.5.1

Router,NATFirewallperaccessoaInternet2 indirizziIPsudueinterfaccediverse

153.18.125.1192.168.5.2

Un altro esempio (cont)


Routerdicampuscheinterconnettele4retiedinoltraalRouterNATdicollegamentoversoInternet:5 indirizziIPtuttisullastessainterfacciafisicadellacampusLAN

168.192.1.1168.192.2.1168.192.3.1168.192.4.1168.192.5.1

Collegamentoalivelloethernet

153.18.125.1168.192.5.2

...

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