Explanation/Reference:
Section: Addressing and Routing Protocols in an Existing Network Explanation Explanation:
You have been instructed to conserve IP address space, so you must create subnets containing a minimal number of IP addresses and still accommodate all necessary hosts. A total of 192 host addresses are required on the network: 25 for Marketing, 55 for Administration, 110 for Research, and two for the serial link connecting the offices. You can create a sufficient number of IP addresses by supernetting a single Class C subnet.
You should begin allocating address ranges starting with the largest group of hosts to ensure that the entire group has a large, contiguous address range available. Subnetting a contiguous address range in structured, hierarchical fashion enables routers to maintain smaller routing tables and eases administrative burden when troubleshooting.
The Research department on Network D requires 110 hosts. A 25-bit subnet mask is sufficient to handle
126 host addresses. Two address ranges with a 25-bit subnet mask are available: the 192.168.1.0/25 and
192.168.1.128/25 networks. Although the 192.168.1.0/25 network would work for Network D, the
192.168.1.128/25 address range is the optimal range for Network D because the 192.168.1.0/25 network range overlaps most of the address ranges of the remaining choices. When addressing the E0/0 interface of Router2 and hosts in the Research department, you should use host addresses from the
192.168.1.128/25 range, which includes addresses from 192.168.1.129 through 192.168.1.254.
The next-largest department, Administration, requires 55 host addresses for Network B.
A 26-bit subnet mask is sufficient to handle 62 host addresses. Of the remaining options, you should use the
192.168.1.64/26 network. When addressing the E1/1 interface of Router1 and hosts in the Administration department, you should use host addresses from the 192.168.1.64/26 range, which includes addresses from 192.168.1.65 through 192.168.1.126.
The smallest department, Marketing, requires 25 host addresses for Network A.
A 27-bit subnet mask is sufficient to handle 30 host addresses. Of the remaining options, you should use the 192.168.1.32/27 network. When addressing the E0/0 interface of Router1 and hosts in the Marketing department, you should use host addresses from the 192.168.1.32/27 range, which includes addresses from 192.168.1.33 through 192.168.1.62.
The point-to-point link between routers requires two host addresses. A 30-bit subnet mask is sufficient to handle exactly two host addresses. Of the remaining options, you should use the 192.168.1.0/30 network.
When addressing the S0/0 interfaces of Router1 and Router2, you should choose host addresses from the
192.168.1.0/30 range, which includes the 192.168.1.1 and 192.168.1.2 addresses.
The overall IP addressing scheme is summarized in the following table:

You should not use the 192.168.2.0/24 network for any of the subnets, because doing so would not conserve IP addresses. A 24bit subnet mask can support 254 hosts in a single broadcast domain and would therefore not conserve address space. A 25bit subnet mask is sufficient to handle the largest subnet in the scenario.
You should not use either the 192.168.1.0/28 or 192.168.1.0/29 networks for any of the subnets. A 28bit subnet mask supports 14 hosts, and a 29bit subnet mask supports six hosts, neither of which is a sufficient number of hosts for the Administration, Marketing, or Research departments. The 28bit and 29bit subnet masks are not appropriate for Network C either, because the number of host addresses created by these masks far exceeds the number of actual hosts on the point-to-point link between Router1 and Router2.
Finally, you should not use the 192.168.1.0/25 network for any of the subnets, because it allows for a single subnet of 126 hosts. In relation to the other network choices available, this subnet mask would not fit the overall addressing schema. If you used this option, the remaining available choices would not meet the design requirement and would not allow for an efficient distribution of addresses.
Reference:
CCDA 200-310 Official Cert Guide, Chapter 8, IPv4 Address Subnets, pp. 302-310 CCDA 200-310 Official Cert Guide, Chapter 8, Plan for a Hierarchical IP Address Network, pp. 311-312.
Cisco: IP Addressing and Subnetting for New Users.