Sign inMonitor a domain
2001::/23

2001

Last updated 2 Nov 2026 from the IANA IPv6 Special-Purpose Address Registry

Short answer

2001::/23 is the IETF protocol-assignments block for IPv6.

Block2001::/23
First – last2001:: – 2001:1ff:ffff:ffff:ffff:ffff:ffff:ffff
Sizea /23 (2^105 addresses)
Name in the registryIETF Protocol Assignments
Defined inRFC 2928
Allocated2000-09
Valid as source / destinationFalse / False
ForwardableFalse
Globally reachableFalse

What it means

RFC 2928 reserves this block for addresses that IETF protocols need, such as Teredo (2001::/32) and benchmarking (2001:2::/48). Individual sub-blocks have their own rules.

An address here usually comes from a tunnelling or test mechanism, not a normal network.

Want the address websites actually see for you? Open the Truth Check.

Sources

  1. IANA IPv6 Special-Purpose Address Registry (CC0) · checked 2 Nov 2026
  2. RFC 2928