256
(each number is the square of the preceding number)
Ports are described by 16 bits. 2^16 is 65536 so the highest number is 65535 and port 65536 does not exist.
49
The sequence consists of perfect squares: (3^2 = 9), (4^2 = 16), (5^2 = 25), and (7^2 = 49). The missing number corresponds to (6^2), which is 36. Therefore, the missing number in the sequence is 36.
65536
65536 virtual ports. Port number is a 16 bit number, so there are 2^16 (65536) ports for a NAT router.
2^16-1= 65536
You could put 49 in between the 16 and the 100.
16
c) 17
9 (between 8 and 16).
22
1111 1111 1111 1111 = 2^16 = 65536