Subnetting IPv6 in Postgres
As 3manuek mentioned, there are functions and operators to deal with IP addresses - be it IPv4 or IPv6.

I am wondering a bit where your number comes from.  The address range you gave translates to 

2001:0dd8:0000:0000:0000:0000:0000:0000


which is an address range of length 1.

If you meant all possible addresses starting with '2001:dd8:', then the correct designation of this range would be 2001:dd8::/32, as it is the first 32 bits that are given.  According to a calculator tool, the resulting range is the following:

Start Range: 2001:dd8:0:0:0:0:0:0
End Range: 2001:dd8:ffff:ffff:ffff:ffff:ffff:ffff
No. of host: 79228162514264337593543950336


Meanwhile, if you omit the block size (/32), you get a single address (the one I showed above).

So, there is a way to get the number provided by the calculator, using builtin PostgreSQL functionality.  Our friend here is masklen():

SELECT round(2 ^ (128 - masklen('2001:dd8::/32'::cidr))::numeric);

             round             
───────────────────────────────
 79228162514264337593543950336


The result is the same as form the calculator.  The rounding is there for removing the decimal point and the zeros after it, which are the result of casting to numeric.