网络路由的原理和常见协议的工作机制

Routing Page1of9What is the Roleof theRouter?Routers interconnectworks inorder to allow munication and resourcesharing amongstthose works.Connections aremade betweenrouters usingcables ora wirelessmedium.Various routing protocols enablethe routersto exchangedata,and can be used to controlthe flowof thatdata.Some routershave switchesbuilt into them,and somerequire a separate switch.A pictureof asmall router can be found below:BEFORE MOVINGFORWARD,IT ISIMPORATANT TOKNOW THATA ROUTERIS NOT A FIREWALLAND AFIREWALL ISNOTAROUTER.THAT SAID,SOME ROUTERSHAVE BUILTIN FIREWALLCAPABILITIES Routing Page2of9How doesa Routeruse Routing Tables?To determihe destinationof apacket,the router will examihe packet and send the packet to theproper destination.To examihe packet,the routerstrips offthe

layer-2information andputs theresulting packetinto aqueue–this processours when the packetarrives at the router.The router will theninspect thedestination IP address of the packetand sendit to the correctport.To performthis,the router will use a routing table.The routing table tellsthe routerexactly where to sendthe packetsfor thegiven destination.An exampleof arouting tablecan be found

below:Destination NetworkSub Mask Gateway

Interface10.12.14.0255.255.255.00.0.0.0LAN76.30.4.0255.255. 254.00.0.0.0WAN0.0.0.00.0.0.076.30.4.1WAN Theponents of the routing table are further described below:1)Destination LAN IP –The definedwork ID.Every work ID directly connected toone of the router’s portsis alwayslisted here.2)Sub Mask–Identifies the work IDfrom anIP address.3)Gateway–The IP address for thehop router(i.e.where the packet shouldgo).If the packet isdestined for a workID thatis notdirectlyconnectedto the router,the gatewaycolumn tells router the IP address of a router to sendthe packetto;the nextrouter handlesthe packetand theoriginal routeris relievedof itsresponsibility forthat packet.If the workIDis directlyconnected,then agateway isn’t needed.4)Interface–The Interfacefield identifieswhich ofits physicalports to use.The providedexample shows“LAN”and“WAN”but somerouters mayshow aport’s IP address oranother typeof abbreviation(like f0/0,f0/1as foundon Ciscorouters).***When lookingatthe routing

table,a0represents“anything.”An exampleof thiscan be found in the providedrouting tablewith theDestination LANIP

of10.12.14.0and thesub maskof255.255.255.0.This meansthat anyvalue between1and254is aeptable.Exercise:Use the routing

tableabove toanswer thefollowing questions.1)What interfacewould apacket receivedwith a Destination LANIP

of76.30.4.27be sent to?________2)If apacket isreceived for aDestination LANIPof69.36.252.61,what Gatewayshould thepacket be

sentto?____________________________________________________ ________________________Routing Page3of9How toIdentify theDefault Routein theRouting TableMost routershave adefault route.A defaultroute tellsthe routerwheretoforward packetswhen noother routecan bedetermined byusing therouting table.In therouting tablebelow,the defaultroute isrepresented in the thirdline:DestinationLANIP SubMaskGatewayInterface10.12.14.0255.255.255.00.0.0.0LAN76.

30.4.0255.255.254.00.0.0.0WAN0.0.0.00.0.0.076.30.4.1WAN Isa Routerthe OnlyThing thathas aRoutingTable?No,every nodeon a work shouldhave arouting table.Take thetime toopen amand promptor terminalwindow andtype the mand stat–r.You mayalso use themandroute printin theWindows mandprompt asan alternativeoption orroute inlinux.The defaultroute.This routingtable showsroutes for10.12.14.0and76.30.4.0;the thirdline in theroutingtable has0.0.0.0for thedestination andsub mask,representing anyother