Generate RFC 4193 unique local IPv6 addresses with custom subnet ID
ULA Options
About IPv6 ULAs (RFC 4193)
What is a ULA?
A Unique Local Address is an IPv6 address (fc00::/7) intended for local communications within a site, similar in spirit to RFC 1918 private IPv4 ranges. The fd00::/8 half is for locally-assigned IDs.
Structure
fd (8 bits) + random Global ID (40 bits) + Subnet ID (16 bits) + Interface ID (64 bits) = 128-bit address. The random Global ID provides probabilistic uniqueness — RFC 4193 recommends generating it randomly so independent sites don't collide when merged.