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OK, now I see, you are subnetting like Doc said.
You should write your IP Address binary.
Start counting at the left hand side.
If the first bit = 0, the Address is Class A.
If the first bit = 1 and the second = 0, the Address is Class B.
If the first and the second bit = 1 and the third = 0, the Address is Class C.
Any IP Address with 129 as first octet is a class B address.
129(10) = 10000001(2)
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