* A customer needs six usable IP addresses, which means they need a subnet that can accommodate at least six hosts, excluding the network address and the broadcast address12.
* The subnet mask determines how many bits are used for the network prefix and how many bits are used for the host identifier12. The more bits are used for the network prefix, the fewer bits are available for the host identifier, and vice versa12.
* The number of usable IP addresses in a subnet can be calculated by subtracting two from the number of possible host identifiers, which is 2^(number of host bits)12.
* For example, a subnet mask of 255.255.255.0 (/24 in CIDR notation) has 24 bits for the network prefix and 8 bits for the host identifier. Therefore, the number of usable IP addresses in this subnet is 2^8 - 2 = 25412.
* The four options in the question have different subnet masks, which correspond to different numbers of usable IP addresses, as shown in the table below:
Table
Subnet mask
CIDR notation
Number of host bits
Number of usable IP addresses
255.255.255.128
/25
7
126
255.255.255.192
/26
6
62
255.255.255.224
/27
5
30
255.255.255.240
/28
4
14
* Therefore, the best option that meets the customer's requirement of six usable IP addresses is C. 255.255.255.224, which has 30 usable IP addresses in the subnet12.
References:
* 1: Subnet Cheat Sheet - 24 Subnet Mask, 30, 26, 27, 29, and other IP Address CIDR Network References
* 2: Network Subnets (CIDR) and Usable IP Addresses Reference Guide