IPv6 ULA generator

Unique Local Addresses (ULAs) are the IPv6 equivalent of private IPv4 ranges like 10.0.0.0/8 and 192.168.0.0/16. They are meant for internal communication and are not routed on the public internet. RFC 4193 requires each network to pick a random 40-bit Global ID so that two organisations are very unlikely to choose the same prefix, which avoids conflicts when networks are merged or connected by VPN. This generator creates such a prefix for you.

How to use it

1. Generate a prefix.
2. Use the resulting /48 for your site and carve /64 subnets out of it.

ULA structure

• Prefix fd (8 bits): fc00::/7 with the L bit set to 1, giving fd00::/8 for locally assigned addresses.
• Global ID (40 bits): random, making your prefix unique in practice.
• Subnet ID (16 bits): up to 65,536 subnets.
• Interface ID (64 bits): the host part.

Example

A generated prefix might be fd12:3456:789a::/48. You could then use fd12:3456:789a:1::/64 for servers, fd12:3456:789a:10::/64 for workstations and fd12:3456:789a:20::/64 for IoT devices.

Why not use fd00::/48?

It is tempting to pick an easy prefix like fd00::, but if every company does that, connecting two networks later leads to address conflicts, exactly the problem ULAs were designed to avoid. Always use a random Global ID.

ULA or global addresses?

Devices can have both. Global addresses reach the internet; ULAs provide stable internal addresses that do not change if your ISP changes your global prefix. They are useful for internal DNS, printers, lab networks and site-to-site VPNs.