the 156 dns is the old secure dns. the ones that start with 8 are the new secure dns that have content filtering. since the new secure dns is still in beta they put the old one as the secondary so if you have any issues it will fall back to the old one or if comodo has any problems with their new servers it will fall back to the old one. its just a precautionary until it comes out of beta. im using the new secure dns for both dns options and i havent had any problems