What's the logical counterpart of the to_hex function?

I have data (from an external source) which is in text format as a hex
number (it's actually an IP address, but that's probably not relevant.)

to_hex gets from integer to hex, I can cast a hex constant to integer (ie
x'12a7'31'::int, but how do I get a database column from string/hex to
integer?

(If this is referenced in the documentation, I'm not finding it.)
--
Mike Nolan