The router’s only recourse is to assume that the mask configured on one of its interfaces attached to 172.25.0.0 is used consistently throughout the network. It will use its own mask for 172.25.0.0 to derive the subnet of the destination address. As the route tables throughout this chapter illustrate, a router that is directly connected to a network will list the network in a heading along with the subnet mask of the connecting interface and will then list all the known subnets of the network. If the network is not directly connected, there is a listing only for the major-class network and no associated mask.
Because the destination addresses of packets being routed by a classful routing protocol are interpreted accor...
How the subnet mask is determined on classful routing
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