IPv4 has 4 billion addresses. IPv6 has 340 undecillion (3.4 × 1038). The transition has been underway since 1998. Most networks now run both; some run only one. Knowing which matters more than the underlying address-space arithmetic.
The format difference
IPv4: 192.0.2.1 — four 8-bit octets, dotted-decimal.
IPv6: 2001:db8::1 — eight 16-bit groups, colon-hex, with consecutive zero groups collapsible to ::.
When IPv6 matters
Modern mobile networks are IPv6-only with CGNAT for IPv4 fallback. Modern cloud APIs accept both. If you're running a public service, publish both A and AAAA records — some clients can't reach you without IPv6 anymore.
Our DNS Lookup tool shows both A and AAAA. IP Lookup works on both protocols.
When IPv4 still wins
Email. The major mail receivers still treat IPv6 mail with more suspicion (newer IP space = less reputation history). Production mail servers should still publish A records and listen on IPv4.