Wireless WZR-600DHP setup questions that are not found in the HELP or User guide

[atlanta_kjd]

Hi, I am new to Buffalo WZR-600DHP router, just bought it yesterday.  Few areas that I do not understand.
I have let the router run the wizard and auto configure itself to use PPOE, there are some areas that I do not understand their usage.
A)
In menu  Setup / Basic Setup -- Under section WAN Setup / Optional Setting -- what does STP enable / Disable does for me?

B)
In menu  Setup / Basic Setup -- Under section Network seup / Router IP the Gateway and Local DNS is set to 0.0.0.0, the router is working fine, but if I want to set the gate to same as router IP, it hands, what does this 0.0.0.0 does?