Choose the right DNS record
A and AAAA map names to IPv4 and IPv6 addresses. CNAME points to another name; MX identifies mail servers. TXT stores verification and email-policy text. NS identifies name servers and SOA describes the zone.
TTL and cached answers
TTL is a cache lifetime in seconds. A recursive resolver may return the remaining lifetime, so repeated lookups can show decreasing values. Different resolvers can temporarily return different cached data after a change.
NXDOMAIN, NODATA and SERVFAIL
NXDOMAIN means the queried name does not exist. NODATA means the name exists but the requested record is absent. SERVFAIL means resolution failed; it does not prove that the domain is missing.
Reverse DNS and email
Entering an IP address automatically requests its PTR record under in-addr.arpa or ip6.arpa. A PTR name is controlled by the address owner and does not prove ownership of a website. Null MX explicitly declares that a domain does not receive email.
Limits of a browser DNS lookup
Requests go to the selected public resolver over HTTPS. This tool cannot inspect your operating system cache, local hosts file, private DNS server or every authoritative server. AD reports validation by the resolver; missing AD alone is not a DNSSEC failure.