Definition
IPv4 is the fourth version of the Internet Protocol, deployed since 1981 and still the most widely used. Its addresses are 32-bit numbers, providing roughly 4.3 billion possible values — a number that seemed inexhaustible at the time and proved deeply inadequate as the internet grew. IANA officially exhausted its pool of unallocated IPv4 addresses in 2011, and regional registries followed; the result is a market where IPv4 blocks are bought, sold, and leased like real estate.
Workarounds for Scarcity
Network Address Translation (NAT) lets many devices share a single public IPv4 address, which is why your home network has many devices but only one external IP. Carrier-Grade NAT (CGNAT) extends this to entire ISP customer bases, putting hundreds or thousands of subscribers behind a single public IP. CGNAT complicates port forwarding, peer-to-peer protocols, and accurate geolocation, which is one reason a dedicated public IP from a VPN can be useful.
VPN Implications
Most VPN servers and clients still operate primarily over IPv4. When you connect through a VPN, your visible address is the VPN's IPv4. If your underlying ISP also assigns IPv6, traffic destined to IPv6 hosts may bypass the IPv4 tunnel and leak — the so-called IPv6 leak. Many VPN clients respond by either tunnelling IPv6 alongside IPv4, or by disabling IPv6 entirely while connected.