Why IPv6 was introduced

1. Address exhaustion (main reason)

• IPv4 has 32-bit addresses, only about $2^{32} \approx 4.3$ billion, which are nearly exhausted because of the growth of the Internet, mobile devices and IoT.
• IPv6 uses 128-bit addresses ($\approx 3.4 \times 10^{38}$), a practically unlimited space.

2. Limitations of IPv4

• Workarounds like NAT and CIDR break end-to-end connectivity and add complexity.
• Complex header with options; routers must fragment and recompute the header checksum at every hop.
• Security (IPsec) was optional.
• Poor support for QoS, real-time traffic, autoconfiguration and mobility.

3. Improvements in IPv6

Feature	IPv4	IPv6
Address size	32 bits	128 bits
Header	Variable, 20-60 bytes, with checksum	Fixed 40 bytes, no checksum
Fragmentation	By routers and source	Only by the source
Configuration	Manual / DHCP	Auto-configuration (SLAAC) plus DHCPv6
Security	IPsec optional	IPsec built in
Broadcast	Yes	No (multicast and anycast)
QoS	Limited (ToS)	Traffic class and flow label

Hence IPv6 solves the address shortage and gives a simpler, faster, more secure and scalable network layer.