IPv6 prefix

A registry gives a provider a large block, most often a /32 or, in the RIPE NCC's region, a /29. The provider gives each customer a prefix out of it. RIPE's guide for providers suggests a /48 for every customer, or a /48 for businesses and a /56 for homes. It calls assigning anything longer than a /56 "strongly discouraged".

The customer's router cuts its prefix into /64s, one for each local network, such as the main network and a guest network. Assigning a customer only a /64 "does not conform to IPv6 standards", in the guide's words, because the customer can then run only one network. Phones are the exception: a phone on mobile data gets one /64 for each data connection.

The first IPv6 addressing standard, of December 1995, expected many devices to use their 48-bit network card address as the device part. The revision of July 1998 made the device part 64 bits, still built from the card's address where one existed. A device therefore carried the same number in its address on every network it joined.

A standard of January 2001 let devices use interface IDs "that change over time" instead. The aim was to make it harder to tell when different addresses "actually correspond to the same node". The 64-bit split itself stayed. In March 2011 the IETF replaced its advice of three fixed customer sizes, /48, /64 and /128. Fixed sizes, it said, raised concerns about "a return to 'classful addressing'".

Many services that limit or block by address therefore count /64s. Cloudflare's previous rate limiting blocked "an individual IPv4 address or IPv6 /64 IP range" once it passed a threshold. When Cloudflare launched IP lists in July 2020, they stopped at /64 because "the lower 64 bits can often be changed by the end user". The lists now take single IPv6 addresses as well.

Spamhaus's exploits blocklist lists IPv6 in /64 blocks, calling a /64 "the industry standard for the smallest IPv6 allocation to individual customers". It adds that listing blocks rather than single /128 addresses makes the tricks spammers use to game a list much harder.

Counting /64s is common, not universal. Invidious, an alternative YouTube front end, is an example. Its documentation says operators could escape YouTube's blocks of 2023 by moving to new addresses inside their own /64.

For IPv6 proxies the prefix matters more than the address count. Open-source scripts turn one /64 on a rented server into proxies on any number of random addresses. A site that counts /64s sees all of them as one visitor, and moving to a new address inside that /64 changes nothing there. Ask how many /64s a pool spans, and how many /48s, before comparing address counts.

Mobile addresses work differently. Over IPv4, a carrier can put many phones behind one shared address with carrier-grade NAT, so one address stands for many people. Over IPv6, each phone's data connection gets its own /64, so one /64 stands for one phone.

Home connections with IPv6 are common. On 18 September 2026, 1,937 of the 2,887 connected RIPE Atlas home probes had an IPv6 address, 67.1 percent. Their hosts are volunteers with an interest in networks, so this is not the share for all homes.