• The reason why IPv4 is not sustainable is because of limited Address Space IPv4 provides only about 4.3 billion unique addresses, which are no longer enough for the billions of internet-connected devices today.

• Widespread Infrastructure and Compatibility – IPv4 is deeply embedded in the internet’s hardware, software, and systems, making it easier and cheaper to keep using than to upgrade.

IPV4 lasted this long due to subnetting and CIDR (Classless Inter-Domain Routing) – These methods helped allocate IP addresses more efficiently and delayed exhaustion.

• Because of Efficient Use of Address Space Techniques like Network Address Translation (NAT) allow multiple devices on a private network to share a single public IP address