IPv6 at a glance
An IPv6 address is 128 bits long. Its left half tells the Internet which network to deliver to, and its right half names one device on that network.
Eight hextets
The 128 bits are written as eight groups of four hex digits, separated by colons. Each group, or hextet, holds 16 bits.
Global routing prefix
The first 48 bits (blue) are assigned through registries and ISPs. Internet routers use only this prefix to send packets toward the right site.
Subnet ID
The next 16 bits (green) belong to the site. Its router uses them to create up to 65,536 separate /64 subnets.
Interface ID
The last 64 bits (orange) identify one interface. Hosts learn the /64 prefix from Router Advertisements and often generate their own interface ID (SLAAC), so no DHCP is needed.
Writing it shorter
Leading zeros in a group can be dropped, and one run of all-zero groups can be replaced by '::'. So the example becomes 2001:db8:85a3::8a2e:370:7334.
A vast address space
IPv4 has about 4.3 billion addresses; IPv6 has about 3.4 × 10^38. That abundance lets every device have a globally unique address and removes the need for NAT.