Data Communications Networking Devices - 4th Ed.pdf

5.2 ROUTERS ____________________________________________________________________ 569to the router connecting the two subnets as if no subnetting existed. The corporaterouter is con®gured via the use of subnet masks to differentiate hosts on one subnetfrom those on the other subnet. From an interior view, packets originating on onesubnet must use the resources of the router to reach hosts on the other subnet aswell as hosts on other networks.Once the transmitting host notes that the destination IP address is on either adifferent network or a different subnet, it must use the services of a router.Although each host will be con®gured with the IP address of the router, the hostwill transport packets via the data link layer, which requires knowledge of the 48-bit MAC address of the router port connected to the segment the transmitting hostresides on.ARPThe translation between IP and MAC addresses is accomplished by the use of theAddress Resolution Protocol ARP). To obtain the MAC address of the router'sLAN interface the host will broadcast an ARP request. This request will bereceived by all stations on the segment, with the router recognizing its IP addressand responding by transmitting an ARP response.Since a continuous use of ARP would rapidly consume network bandwidth,hosts normally maintain the results of ARP requests in cache memory. Thus, oncethe relationship between an IP address and a MAC address is learned, subsequentrequests to transmit additional packets to the same destination can be accomplishedby the host checking its cache memory.When packets arrive at the router destined for a host on one of the subnets, asimilar process occurs. That is, the router must obtain the MAC addresses associatedwith the IP address to enable the packet to be transported by data link layerframes to its appropriate destination. Thus, in addition to being able to correctlysupport the transmission of packets from one interface to another, an IP-compatiblerouter must also support the ARP protocol. Later in this section we willdiscuss and describe additional protocols routers can support.Communications and routing protocolsFor routers to be able to operate in a network they must normally be able to speakthe same language at both the data link and network layers. This means that therouters must communicate with one another using the same route protocol as wellas the same routing control protocol.Routing protocolsExamples of route protocols include Transmission Control Protocol and theNetwork Protocol TCP/IP), Xerox Network Services Network Transport ProtocolXNS), Digital Equipment Corporation's DECnet, Novell's IPX, and Apple