QUESTION 3 266 Chapter 8 TCP/IPInternetworking I Packet arrives with destination

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QUESTION 3

266 Chapter 8 TCP/IPInternetworking I Packet arrives with destination IP address 128.171.17.13 Decide how to send the packet back out Select the best-match row Find all row matches MetricInterfaceNext (Cost) RowDestination Mask /Prefix) Network or Subnet 128.171.0.0 172.30.33.0 60.168.6.0 123.0.0.0 172.29.8.0 172.40.6.0 128.171.17.0 172.29.8.0 172.12.6.0 172.30.12.0 172.30.12.0 60.168.0.0 0.0.0.0 47 2 255.255.0.0 (16) 255.255.255.0 (/24) 255.255.255.0 (24) 255.0.0.0 V8) 255.255.255.0 /24) 255.255.255.0 /24) 255.255.255.0 (24) 255.255.255.0 (/24) 255.255.255.0 (/24) 255.255.255.0 (/24) 255.255.255.0 (/24) 255.255.0.0 /16) 0.0.0.0 0) 12 34 47 20 10 12 16 13 FIGURE 8-11 Routing Table However, there are many more IPv4 addresses on the Internet than there are Eth ernet addresses on an Ethernet network. Even with rows representing groups of IPv4 addresses, core routers in the Internet backbone still have several hundred thousand rows. This is important. We will see that routers need to do calculations for all rows. Test Your Understanding 8. a) In a routing table, what does a row represent? b) Do Ethernet switches have a row for each individual Ethernet address? c) Do routers have a row for each individual IPv4 address? d) What is the advantage of the answer to the previous subparts of this question? Step 1: Finding All Row Matches We will now see how the router uses its routing table to make routing decisions. Figure 8-12 shows that there are three very different steps. These differences can lead to confusion, so you must study this material carefuliy. The first step is to find

Explanation / Answer

For the packet with ip address 172.30.12.124, the corresponding destination network/subnet are on row 10, 11 and 13 (assuming default). Among these, the destination with the least cost involved as mentioned in metric column must be chosen. For the given gase it is the destination network 172.30.12.0 (on row 11) is the least costly (metric cost 3) among the other destinations. Hence the packet must be forwarded on corresponding inteface 3 to next hop router H.

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