Ender204426777乾颐堂CCNAv3.0课程EstablishingInternetConnectivityExploringthePacketDeliveryProcess探索报文的转发乾颐堂CCNAv3.0Ender204426777Layer2AddressingLayer2characteristics:•EthernetusesMACaddresses.•IdentifiesenddevicesintheLAN.以太网用MAC地址标示局域网中的唯一性•Enablesthepackettobecarriedbythelocalmediaacrosseachsegment.MAC地址表帮助同一网段的设备转发数据，数据包直接穿越它EnderICND1—2-86乾颐堂CCNAv3.0Ender204426777Layer2Addressing(Cont.)Layer2addressing:•TherouterhastwointerfacesdirectlyconnectedtotwoPCs(去往帝都的ID)•EachPCandeachrouterinterfacehasitsownuniqueMACaddress.每个PC和路由器的每个接口都有唯一的MAC地址L2=Layer2EnderICND1—2-87乾颐堂CCNAv3.0Ender204426777Layer3AddressingLayer3devicesandfunctions:•Thenetworklayerprovidesconnectivityandpathselectionbetweentwohostsystems.网络的路由提供了两台主机的路径选择（去往美国需要护照）•Inthehost,thisisthepathbetweenthedatalinklayerandtheupperlayers.•Intherouter,itistheactualpathacrossthenetwork对路由器它是真实的穿越网络的路径EnderICND1—2-88乾颐堂CCNAv3.0Ender204426777Layer3Addressing(Cont.)Layer3addressing:Layer3addressesmustincludeidentifiersthatenableintermediarynetworkdevicestolocatehostsondifferentnetworks.IP地址用来定位主机和网络的位置•TCP/IPprotocolstackusesIP.通过IP地址把数据包路由到某个网络，MAC地址是不能跨越网段的EnderICND1—2-89乾颐堂CCNAv3.0Ender204426777Layer3Addressing(Cont.)•Layer3addressesareassignedtohostsandnetworkdevicesthatprovideLayer3functions.IP地址指定给主机和网络设备提供三层路由功能•Networkdevicesmaintainaroutingtable.RoutingTable192.168.3.0/24InterfaceGi0/0192.168.4.0/24InterfaceGi0/1L3=Layer3EnderICND1—2-90乾颐堂CCNAv3.0Ender204426777AddressResolutionProtocolARPprovidestwobasicfunctions:•ResolvingIPaddressestoMACaddresses解析三层地址到二层地址•Maintainingacacheofmappings维护该映射的缓存EnderICND1—2-91乾颐堂CCNAv3.0Ender204426777ARP协议172.16.3.1172.16.3.2IP:172.16.3.2=???我需要知道172.16.3.2的物理地址.乾颐堂CCNAv3.0Ender204426777ARP协议172.16.3.1172.16.3.2IP:172.16.3...
