Converting a ipv4 multicast to ipv6. Please help!!

How to convert 224.0.0.5 to ipv6:

Why does 224 convert to ff in ipv6?

My thinking is:

 224 0 0 5 

1110 0000 | 0000 0000 | 0000 0000 | 0000 0101

ff 02 0 5

So the answer is: ff02::5 but I don't understand why. When I break up the ff I do this: 1111 1111 - which gets me 255. The ff is referring to the 1st octet for this problem in ipv4 addressing? Why am I wrong?

Please explain in binary, I prefer to learn this way. Thank you.