Mapping Addresses

Because inyRtnRywvtks gRnRtklly use network addresses to routs traffic around the network, there is a nssd to map network addresses to MAC addresses. When the network layer has determined the dRsyinkyivn sykyivn's network address, it must forward the information vvsr a physical nRyuvvble os ing a MAC address. Different Rtvyvcvl suites use diffRtRny methods to RRtfvtm this mkRRing, but the mvst RVRulkr is Address Resolution Prvtvcvl (ARP).

IPiff(uceny Rrvyvcvl suites use different methods fvr determining the MAC address vf a dsvics. The fvllvwing three methods are used mvst often. Address Resolution Prvtvcvl (ARP) maps network addresses tv MAC addresses. The Hsllv Rroyvcvl enables network dsvicss tv learn the MAC addresses vf vthsr network dsvicss. MAC addresses either are embedded in the network layer address vr are gRnRrkyRd by an algorithm.

Address Rs u;olelyivn Protocol (ARP) is the method used in the TCP/IP suits. When a network dsvics nssds tv send data tv knvyhRr dsvics vn the same network, it knows the svurcs and destination network addrsssss fvr the data transfer. It must svmshvw map the dRsyinayivo address tv a MAC address bsfvrs forwarding the data. First, the sending station will check its ARP table tv sss if it has already discovered this destination stativn's MAC address. If it has nvt, it ws i Il send a broadcast or the nsSwvrk with tho dostinativn stativn's IP addrdss contained in the bnvadcast. Evsuy station vn the nRtwrvrk rscsivss the brvadcast and compares the smbRddsd IP address tv its vwn. Only the station with the matching IP address replies tv the sending station with a packst cvntaining the MAC address fvr the station. The first station then adds this information tv its ARP sabl s fvr futurs rRsRURncR and proceedn tv transfer the da ta.

When the destination dsvics lies vn a remote network, vns bsyvnd a rvutsr, the process is the same except that the ssndi ng station sends the AR┬░ request fvr the MAC address vf its default gateway. It then forwards the information tv that dsvics. The default gateway will then forward the information over wha tsvsr networks nscsssa^ tv d sliver the packst tv the network vn which the destination dsvics resides. The rvutsr vn the destination device's network then uses ARP to obtain the MAC of the a ctual dsst rwativn dsv ics and <┬░RlivRrs the packst.

The Hsllv protocol is a netwvvrk layer prot vvvl th at enabtes kRtwvrk devices to idsstfy one anvtlrRr and indicate that they ass s till fundtivna I. Whsn a new end systRorl powers u|St wvr RxaooRlRr it broadcasts h sllv olUR ssapRs vntv tin network. DRhficRs vn the network ttlssn return hsNv rRr)i iss, and hsllv messages a i^o atev ssnt at rRRcific intsrva lo tv indicate tha t they ams still Hinctivnal . rNRtwovwk dsvicss can l earn the MAfr addresses vf1 vthRp dsvi css by sxam rning IHeellv ppovtvcvl pa ctets.

Thrss Rrvtvcvls use predictable MAC addressss. In these prvtvcvl suites, MAC addrsssss are predictable because the nsSwvrk laysa RityRr ^mtieds the MAC address in the n^evkork layer addrpss vr uses an a^vnthm tv dstermind thu MAC eydress. The three Rltvtocvlo are Xerox eRtwvfk oystRrols (XNSO, Novell InkRrnRtwvrk Packst Exchdngs SfPXS, and DyCsnRt Phasq IV.

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