may someone correct me pertaining the subneting concept.
from above, means my ip address will start from 172.17.16.x till 172.17.17.x
start from 172.17.15.x till 172.17.16.x
It all depends on the subnet mask which you have. In your case you mask was 255.255.254.0 so it can start like 172.17.20.x and will go till 172.17.21.x and next can be 172.17.22.x till 172.17.23.x and so on.
But if I take the same example with mask as 255.255.248.0 then it will not start from 172.17.20.x then it will start from 172.17.16.x and will go till 172.17.23.x
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Your first option is correct.
Ip address will start from 172.17.16.x till 172.17.17.x