Why IPv6
IPv4 has about four billion addresses, which the internet exhausted. IPv6 expands the address to one hundred twenty eight bits, an effectively unlimited supply. Addresses are written as eight groups of four hexadecimal digits separated by colons, with runs of zeros collapsed.
What changed
- Huge address space removes the need for widespread network address translation.
- Simplified header is fixed in size and pushes options into extension headers, easing router processing.
- No in transit fragmentation so the source must size packets with path MTU discovery.
- Stateless autoconfiguration lets a host build its own address from a router advertised prefix.
IPv6 also replaces ARP with neighbor discovery, which uses ICMPv6 messages to find link layer addresses and detect duplicates. Adoption is gradual, so most networks run IPv4 and IPv6 together in a dual stack, where a host holds both kinds of address and chooses based on what the destination supports.
Key idea
IPv6 uses one hundred twenty eight bit addresses to escape IPv4 exhaustion and adds autoconfiguration and a simpler header.