There are five to six penetrations in the water heater tank. Each penetration has steel threads. The area immediately surrounding the threads is unprotected by the porcelain lining of the tank. Each one of these connections has an item threaded into it which has metal that may react with the steel body of tank, especially around that threaded penetration including the threads.
Penetrations are for the water heater cold water inlet, the hot water Outlet, the outlet for the drain plug / anode rod, the relief valve Outlet, the gas control / temperature sensor. There may also be a electric heating element which is screwed into the tank.
All of these require protection by the anode rod. I have a Suburban water heater that was leaking through the gas control which also has a copper thermometer well that extends into the tank for the temperature sensor. I personally wouldn't go more than a few weeks without installing a new anode.