6,48,64,800 ways 

There are total 16 pieces which can be arranged on 16 
places in 16P16 = 16! ways. 
(16! = 16 * 15 * 14 * 13 * 12 * ..... * 3 * 2 * 1) 

But, there are some duplicate combinations because of 
identical pieces. 
?	There are 8 identical pawn, which can be arranged 
in 8P8 = 8! ways. 
?	Similarly there are 2 identical rooks, 2 identical 
knights and 2 identical bishops. Each can be arranged in 
2P2 = 2! ways.
Hence, the require answer is 
= (16!) / (8! * 2! * 2! * 2!) 
= 6,48,64,800