Need simple algorithm for newbie

[Jason Tudisco]

I have a list of domains... Some of the domain names in the list look
like this:

groups.goodle.com

The information I want is just google.com. I need to know the best way
to do this.. for .com .net .org only.. and to strip the rest of the
garbage.. like in this case.. get rid of groups in groups.google.com

I need to parse though a huge list so it has to be optimized algorithm

No need to write complete code.. Just get me in the right direccion..
Still learning python and I am not sure what would be the fastest way
to go about it..