IPv6 (Internet Protocol Version 6)

IPv6 (Internet Protocol version 6) is a set of specifications from the Internet Engineering Task Force (IETF) that’s essentially an upgrade of IP version 4 (IPv4). The basics of IPv6 are similar to those of IPv4 — devices can use IPv6 as source and destination addresses to pass packets over a network, and tools like ping work for network testing as they do in IPv4, with some slight variations.The most obvious improvement in IPv6 over IPv4 is that IP addresses are lengthened from 32 bits to 128 bits. This extension anticipates considerable future growth of the Internet and provides relief for what was perceived as an impending shortage of network addresses. IPv6 also supports auto-configuration to help correct most of the shortcomings in version 4, and it has integrated security and mobility features.

IPv6 features include:

Supports source and destination addresses that are 128 bits (16 bytes) long.
Requires IPSec support.
Uses Flow Label field to identify packet flow for QoS handling by router.
Allows the host to send fragments packets but not routers.
Doesn’t include a checksum in the header.
Uses a link-local scope all-nodes multicast address.
Does not require manual configuration or DHCP.
Uses host address (AAAA) resource records in DNS to map host names to IPv6 addresses.
Uses pointer (PTR) resource records in the IP6.ARPA DNS domain to map IPv6 addresses to host names.
Supports a 1280-byte packet size (without fragmentation).
Moves optional data to IPv6 extension headers.
Uses Multicast Neighbor Solicitation messages to resolve IP addresses to link-layer addresses.
Uses Multicast Listener Discovery (MLD) messages to manage membership in local subnet groups.
Uses ICMPv6 Router Solicitation and Router Advertisement messages to determine the IP address of the best default gateway.

This YouTube video by makkyosky provides an entry-level overview of why we need IPv6.