Answer:
175.99.136.0
Explanation:
175.99.138.0/21
Step 1: convert both the ip and the subnet mask to binary format
175.99.138.0 - 10101111.1100011.10001010.00000000
/21 : 255.255.248.0 - 11111111.11111111.11111000.00000000
step 2: perform the bitwise AND operation
In an AND operation, a result of 1 can only be gotten when both inputs are 1. Otherwise, the result will be 0.
10101111.1100011.10001010.00000000
11111111. 11111111. 11111000. 00000000
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10101111. 1100011. 10001000. 00000000
step 3: convert the binary result back to base 10 number
10101111. 1100011. 10001000. 00000000
175.99.136.0