Final answer:
The correct answer is option b. 10.119.128.1, which is obtained by finding the network address for the subnet and then adding one to determine the first host address.
Step-by-step explanation:
The correct answer is option b. 10.119.128.1. To find the first valid host address in a subnet, you need to identify the network address and then determine the first host address by adding one to the last octet of the network address. A /20 subnet mask corresponds to 255.255.240.0, which means the first 20 bits are the network part and the remaining 12 bits are for host addresses. Start by converting the subnet mask to binary, align it with the IP address, and perform a bitwise AND operation to find the network address.
The IP address 10.119.136.143 in binary is 00001010.01110111.10001000.10001111, and the subnet mask /20 is 11111111.11111111.11110000.00000000. After applying the bitwise AND operation, we get the network address as 00001010.01110111.10000000.00000000, which translates back to 10.119.128.0. Therefore, the first valid host is one more than the network address, which is 10.119.128.1.