Final answer:
Given an IP address of 220.42.56.20 with a /26 subnet mask, the correct network definition is option A, 220.42.56.0, because the subnet mask corresponds to a range increment of 64 and 56 falls within the first range of the fourth octet.
Step-by-step explanation:
When you are given an IP address like 220.42.56.20 with a subnet mask of /26, you need to determine the network address that this IP falls under. A subnet mask of /26 indicates that the first 26 bits of the IP address are used for the network portion, and the remaining bits are used for host addresses within that network. In binary, the subnet mask /26 corresponds to 255.255.255.192. This means that the increment for the fourth octet is 64 (256 - 192), thus defining the ranges for networks.
To find the network definition, take the fourth octet and find out where its value fits in terms of multiples of 64. Since 56 is less than 64, the network address is indeed 220.42.56.0. Therefore, with a /26 subnet mask, the correct network definition for the IP address 220.42.56.20 is option A, 220.42.56.0.