Explanation/Reference:
Section: Addressing and Routing Protocols in an Existing Network Explanation Explanation:
Of the available choices, the 172.31.148.0/24 network address and subnet mask combination summarizes the smallest network. The /24 notation indicates that a 24-bit subnet mask (255.255.255.0) is used. A 24- bit subnet is typically used to represent a single Class C, 256-host subnet. In this scenario, the
172.31.148.0/24 network is a 256-host subnet of the Class B 172.31.0.0 network. The 256-host subnet's network address is 172.31.148.0. Its broadcast address is 172.31.148.255. In its classful form, this network would be represented by a /16 subnet mask (255.255.0.0) and contain a total of 65,534 hosts. The Class B range network address would be 172.31.0.0. The broadcast address would be 172.31.255.255.
The 172.20.3.0/21 network address and subnet mask combination does not summarize the smallest network. The /21 notation indicates that a 21bit subnet mask (255.255.248.0) is used, which can summarize two /22 networks, four /23 networks, eight /24 networks, and so on. In this scenario, the
172.20.3.0/21 subnet results in a subnet that can support 2,046 hosts. This subnet's network address is
172.20.0.0. Its broadcast address is 172.20.7.255. The next subnet of addresses from the Class B range would thus have a network address of 172.20.8.0. If this subnet were also a /21, it would have a broadcast address of 172.20.15.255.
The 172.16.2.0/16 network address and subnet mask combination does not summarize the smallest network. The /16 notation indicates that a 16bit subnet mask (255.255.0.0) is used. This subnet mask in fact encompasses the entire classful 172.16.0.0 network range. Therefore, the network address of the
172.16.2.0/16 network is 172.16.0.0. Its broadcast address is 172.16.255.255, for a total of 65,534 hosts.
The 172.16.1.0/8 network address and subnet mask combination does not summarize the smallest network. The /8 notation indicates that an eightbit subnet mask (255.0.0.0) is used. This subnet mask encompasses a range of 16,777,214 IP addresses. For example, the Class A 10.0.0.0 network is a /8 range of IP addresses. In this scenario, the 172.16.1.0/8 network would include every address in the range from 172.0.0.0 through 172.255.255.255.
Reference:
CCDA 200-310 Official Cert Guide, Chapter 8, IPv4 Address Subnets, pp. 302-310 Cisco: IP Routing Frequently Asked Questions: Q.
What does route summarization mean?
Cisco: IP Addressing and Subnetting for New Users