How big 128 bits is
IPv4's 32 bits give about 4.29 billion addresses, which sounded like plenty in the early days and wasn't. IANA's free pool ran out on January 31, 2011, and the Asia Pacific registry APNIC ran out that April. IPv6's 128 bits give about 340.28 undecillion addresses (an undecillion is a 1 followed by 36 zeros).
Adoption has been slow but it's real. In April 2026 APNIC reported that around half of the Internet users visible to Google already reach its services over IPv6.
Reading an address
An IPv6 address is eight groups of four hex digits, like 2001:0db8:0000:0000:0000:0000:0000:0001. Leading zeros in a group can go, and one run of all zero groups can be replaced by ::, once per address. That address becomes 2001:db8::1.
The first 64 bits are normally the network prefix and the last 64 the interface ID, because RFC 4291 requires 64 bit interface IDs for most unicast addresses. It uses the same slash notation as IPv4, so a prefix like 2001:db8::/48 means the first 48 bits are fixed.
Subnetting without counting hosts
One LAN is a /64, which holds 18,446,744,073,709,551,616 addresses (18.45 quintillion). Nobody sizes it to fit the devices, since you won't run out. What you plan is how many /64 networks you need.
RFC 6177 says an end site should get at least one /64 and in most cases significantly more, and gives a /56 as an example default for homes. A /56 holds 256 LANs and a /48 holds 65,536. Azure only accepts IPv6 subnets of exactly /64.
What disappears
IPv6 has no broadcast address. Multicast replaces it, and ff02::1 reaches every IPv6 node on the local link. Subnet masks in dotted form are gone too, since the prefix length says it all.
Routers along the path no longer fragment packets either. RFC 8200 leaves fragmentation to the sending device.