Because these routers only know about their directly connected networks,the routers can only communicate with those devices on their own directly connected LANs and serial networks.
For example, PC1 in the topology has beenconfigured with the IP address 172.16.3.10 and the subnet mask 255.255.255.0. PC1 has also been configured with the default gateway IP address 172.16.3.1, which is therouter's FastEtherent 0/0 interface IP address. Because R1 only knows about directly connected networks, it can forward packets from PC1 to devices on the172.16.2.0/24 network, such as 172.16.2.1 and 172.16.2.2. Packets from PC1 with any other destination IP address, such as PC2 at 172.16.1.10, would be dropped by R1.Let's take a look at the routing table for R2 in the figure. R2 only knows about its three directly connected networks. Try to predict what will happen if we ping oneof the FastEthernet interfaces on one of the other routers.
Click ping in the figure.
Notice that the pings failed, as indicated by the series of fiveperiods. It failed because R2 does not have a route in its routing table that matches either 172.16.3.1 or 192.168.2.1, which is the ping packet's destination IPaddress. To have a match between the packet's destination IP address of 172.16.3.1 and a route in the routing table, the address must match the number of left-most bitsof the network address as indicated by the prefix of the route. For R2, all the routes have a /24 prefix, therefore, the left-most 24 bits are checked for each route