2 to the power of 256 - 1 in my Excel yields 1.16e+77 and yes it does look like some arbitrary number...
Excel doesn't have bignum support. Python does.
>>> 2**256-1
115792089237316195423570985008687907853269984665640564039457584007913129639935L
>>> hex(_)
'0xffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffL'