Explanation/Reference:
Section: Addressing and Routing Protocols in an Existing Network Explanation Explanation:
You should issue the ip route 172.16.0.0 255.255.248.0 172.16.12.1 command on RouterB to configure static routing appropriately in this scenario. The ip route command uses the syntax ip route net-address mask next-hop, where net-address is the network address of the destination network, mask is the subnet mask of the destination network, and next-hop is the IP address of a neighboring router that can reach the destination network. In this scenario, you should choose a static route to the eight displayed networks.
Static routes are also optimal for routing networks that do not change often. Although you could issue an ip route command for each of the eight networks connected to RouterA, this is not the most desirable approach, because, consequently, the routing table on RouterB would become unnecessarily large. This in turn would cause the routing table to consume excessive random access memory (RAM) and CPU resources on RouterB. The correct approach is to use a summary address and subnet mask that encompass all of the desired networks on RouterA in a single ip route command.
The diagram shows that the first of the eight networks on RouterA is 172.16.0.0/24. Eight networks can be summarized in 3 bits (23 = 8). You can write a network statement that encompasses the eight networks that begin with 172.16.0.0/24 by taking 3 bits away from the subnet mask. Moving the 24bit mask 3 bits to the left yields a 21bit mask, which is 255.255.248.0. Thus the network and subnet mask combination of
172.16.0.0 255.255.248.0 encompasses all eight networks. The process of taking bits away from the subnet mask to more broadly encompass additional networks is called supernetting. This is the opposite of subnetting, which is a technique used to divide a network into smaller subnetworks.
You should not issue the ip route 172.16.0.0 255.255.0.0 172.16.12.1 command on RouterB. This would cause RouterB to attempt to forward traffic to all networks from 172.16.0.0/24 through 172.16.255.0/24 to RouterA. The eight desired networks on RouterA would be forwarded, but many additional networks that you may not want to forward to RouterA would also be included.
You should not issue the ip route 172.16.0.0 255.255.240.0 172.16.12.1 command on RouterB. This would cause RouterB to attempt to forward traffic to all networks from 172.16.0.0/24 through 172.16.15.0/24 to RouterA. The eight desired networks on RouterA would be forwarded, but eight additional networks from
172.16.8.0/24 through 172.16.15.0/24 that you may not want to forward to RouterA would also be included.
You should not issue the ip route 172.16.0.0 255.255.224.0 172.16.12.1 command on RouterB. This would cause RouterB to attempt to forward traffic to all networks from 172.16.0.0/24 through 172.16.31.0/24 to RouterA. The eight desired networks on RouterA would be forwarded, but 16 additional networks from
172.16.8.0/24 through 172.16.31.0/24 that you may not want to forward to RouterA would also be included.
You should not issue the ip route 172.16.0.0 255.255.255.0 172.16.12.1 command on RouterB. This would cause RouterB to attempt to forward traffic for only the 172.16.0.0/24 network and would not forward traffic for the other seven networks that are connected to RouterA.
Reference:
CCDA 200-310 Official Cert Guide, Chapter 8, IPv4 Address Subnets, pp. 302-310 Cisco: IP Addressing and Subnetting for New Users