What is the analogy of AD 6?

Perhaps you are referring to CE (Common Era), which in some circles has replaced AD (in the year of our Lord). If so, then AD 6 would be CE 6. (The more usual forms are 6 AD and 6 CE.) Those who use AD for years after Christ's birth will use BC for years Before Christ. Those who prefer CE will use BCE (before the Common Era) in place of BC. The numbers in the two systems are the same.