IPv6 ULA Prefix Generator
Generate a random /48 IPv6 unique-local prefix (RFC 4193).
Overview
The IPv6 ULA prefix generator creates a random /48 Unique Local Address prefix per RFC 4193 — the IPv6 equivalent of an RFC 1918 private range. Click generate and the tool produces a prefix like fd0a:b1c2:d3e4::/48, along with several /64 subnet suggestions and the matching reverse-DNS zone.
Home lab operators carving out an IPv6 private network, enterprise architects planning an internal-only segment, and lab instructors building disposable topologies all need a ULA generator. Long-tail keywords covered: generate random IPv6 ULA prefix, RFC 4193 unique local address example, and pick /64 subnets from a /48 ULA.
How it works
ULAs occupy the fc00::/7 block, but in practice the fd half (fd00::/8) is the only one with a defined algorithm — fc00::/8 is reserved for a future centrally-allocated scheme that has not arrived. The remaining 40 bits between the fd prefix and the /48 boundary are the global ID, which should be picked at random rather than chosen by hand. RFC 4193 specifies an SHA-1 over a timestamp plus the EUI-64 MAC of the originating interface, but in practice a cryptographically random 40-bit value is universally accepted.
A /48 gives you 65,536 /64 subnets — far more than any home network or small enterprise will ever consume. Each /64 then supports the full SLAAC autoconfiguration scheme for hosts on that segment. ULAs route freely inside your network but should not be advertised to the public internet.
Examples
fd0a:b1c2:d3e4::/48— a generated /48 with 40 random bits, ready for internal use.- First
/64subnet:fd0a:b1c2:d3e4:0000::/64, the convention for the main LAN. - VLAN subnet 5:
fd0a:b1c2:d3e4:0005::/64. - Reverse DNS zone:
4.e.3.d.2.c.1.b.a.0.d.f.ip6.arpa— what you delegate inside your network for PTR records.
FAQ
Why not just make up a prefix by hand?
Hand-picked prefixes collide when networks merge. If two sites both use fd00:0:0::/48 and connect via VPN, the addresses overlap exactly like overlapping 192.168.1.0/24 ranges. A random 40-bit ID makes a collision vanishingly unlikely.
Should ULAs ever leave my network?
No. Border routers and firewalls should drop ULA-sourced traffic at the egress, the same way they would drop RFC 1918 IPv4. Public services must use global unicast addresses.
Can I combine ULAs with global addresses?
Yes — hosts often hold both. Modern stacks prefer the global address for internet destinations and the ULA for internal targets via source-address selection rules (RFC 6724).
Is the generated prefix really unique?
40 bits of randomness gives an astronomical collision space. For all practical purposes a random ULA is unique without coordination.