Address exhaustion (only ~4.3 billion IPv4 addresses), the explosion of Internet-connected devices (IoT), and NAT breaking true end-to-end connectivity.

Three Major IPv4 Issues:

IPv4 Exhaustion Timeline:

• IANA pool exhausted: February 2011
• APNIC (Asia-Pacific): April 2011
• RIPE NCC (Europe): September 2012
• LACNIC (Latin America): June 2014
• ARIN (North America): September 2015

Context: IPv6 provides 340 undecillion addresses (3.4 × 10^38) compared to IPv4's 4.3 billion - enough for every grain of sand on Earth to have billions of addresses.

Go deeper:

• IPv4 address exhaustion (Wikipedia) — the depletion story with the exact IANA (2011) and per-RIR exhaustion dates.
• IPv6 (Wikipedia) — why the IETF designed a 128-bit successor and what else it fixes beyond address space.

Dual stack (run both protocols at once), tunneling (encapsulate IPv6 inside IPv4), and translation/NAT64 (convert between IPv4 and IPv6).

IPv4/IPv6 Coexistence Methods:

Dual Stack:

• Preferred method during transition
• Device has both IPv4 and IPv6 addresses
• Uses IPv6 when available, falls back to IPv4
• Most modern OS support dual stack

Tunneling:

• Carries IPv6 packets across IPv4-only infrastructure
• IPv6 packet becomes payload of IPv4 packet
• Common protocols: 6to4, ISATAP, GRE tunnels

Translation (NAT64):

• Allows IPv6-only devices to communicate with IPv4-only devices
• Similar to IPv4 NAT
• Useful when endpoints can't be dual-stacked

Go deeper:

• IPv6 transition mechanism (Wikipedia) — catalogues dual-stack, the tunnelling families (6to4, 6rd, ISATAP, Teredo) and NAT64/DNS64 translation.