When determining the default subnet mask of an IP you need to identify which class (A, B, C, etc.) the IP falls under. The first octet of the IP is 172 which falls in the class B range of 128 – 191. The corresponding default subnet for this range is 255.255.0.0. A network architect is not required to use the default and could design the network to use any subnet mask, but the question specifically asks for the default mask.