PostgreSQL type extensions for IP and MAC addresses.
I needed to record IP and MAC level ethernet addresses in a data
base, and I really didn't want to store them as plain strings, with
no enforced error checking, so I put together the accompanying code
as my first experiment with adding a data type to PostgreSQL. I
then thought that this might be useful to others, both directly and
as a very simple example of how to do this sort of thing, so here
it is, in the hope that it will be useful.
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