NAT stands for natural Address Translation. I remember way back when DSL first came to my area, you had to edit the NAT tables for games like Diablo to work. Since then I think most of that has been fixed. I would leave it at default unless you start having some problems.
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People don't like to be meddled with. We tell them what to do, what to think, don't run, don't walk. We're in their homes and in their heads and we haven't the right. We're meddlesome. ~River Tam
Sedao