Given IPA $\in C_B$ in which $16$ bits is NID, $16$ bit is HID. out of $16$ bits of HID $10$ bits is used as SID as $\log_2600=10$.
to make a subnet mask all NID and SID bits are set to $1$, and HID bits are set to $0$.
So subnet mask=$11111111.\ 11111111.\ 11111100.\ 00000000=255.255.252.0$