In binary, there is 8 bits in an octet. We are now going to half the octet and turn it into 4 bits (also known as a nibble). So our total maximum value is 1111, which in decimal equates to 15 (8+4+2+1). Another example is 1110 in binary, converted to decimal equates to 14 (8+4+2+0 = 14).

Hex value FE80 converted to Binary = 1111111010000000. How did I convert that without a calculator?

Each value in hex is worth 4 bits (1 nibble). What we do is convert it to decimal, then convert them to binary, as shown in the table below:

Hex Decimal Binary
F 15 1111
E 14 1110
8 8 1000
0 0 0
Put all the binary numbers together in the order of FE80, and you get 1111111010000000.