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.)
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Mike Nolan