IPv6 ULA Generator

O que é IPv6 ULA?

ULA (Unique Local Addresses, RFC 4193) é o equivalente IPv6 dos blocos privados IPv4 (192.168.x.x, 10.x.x.x). Faixa: fc00::/7 — na prática fd00::/8, com bit "L" igual a 1 indicando "atribuição local".

Os 40 bits seguintes (Global ID) devem ser aleatórios para minimizar colisão se duas redes ULA forem unidas. Esta ferramenta segue exatamente isso.

Cada prefixo /48 oferece 65.536 sub-redes /64.

Unique Local Addresses in depth

ULA stands for Unique Local Address and is defined in RFC 4193 (October 2005). It is the IPv6 analogue of the private IPv4 ranges from RFC 1918 (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16): addresses meant for internal use that are not routed on the public internet. The full ULA block is fc00::/7, whose first seven bits are 1111110. The eighth bit is the "L" flag: fc00::/8 is reserved for a never-implemented centrally-coordinated scheme, while fd00::/8 — the only half actually in use — is for locally assigned, randomly generated prefixes.

The layout of a ULA address is 7 bits prefix + 1 bit L flag + 40 bits Global ID + 16 bits Subnet ID + 64 bits Interface ID. RFC 4193 §3.2.2 specifies how to compute the Global ID: take the current NTP timestamp (64 bits), append the EUI-64 of any local interface, hash the concatenation with SHA-1 and keep the lowest 40 bits. This produces enough entropy that, when two ULA-using organizations merge networks, the probability of a collision per pair of /48 prefixes is roughly 2^-40 — a problem RFC 1918 IPv4 simply cannot solve, because everybody picks 10.0.0.0/8 or 192.168.1.0/24.

When to use ULA

ULA is the right tool for lab and home networks, site-to-site VPNs, IoT deployments, Kubernetes/Docker pod networks and anywhere you need stable internal addresses that survive ISP changes and don't require NAT to reach each other. ULA coexists with Global Unicast Addresses (GUA): a typical dual-stack host gets a GUA from the ISP for internet traffic and a ULA for intranet traffic that must keep working even when the WAN link is down.

FAQ

Can I announce a ULA prefix on the public internet? No. RFC 4193 §4.1 forbids advertising ULA outside the site and every well-configured ISP filters fc00::/7 at the edge. Packets sourced from a ULA will be dropped well before reaching their destination.

Should I use NAT66 with ULA? Rarely. The whole point of ULA is to avoid the address-translation hacks that defined IPv4. If you need to reach the internet from a ULA-only host, run dual stack and assign a GUA too. NAT66 exists for niche cases (multihoming without PI space) but is widely considered an anti-pattern.

How is ULA different from Link-Local (fe80::/10)? Link-Local addresses are confined to a single L2 segment — routers never forward them. ULA addresses are routable inside your site/AS across multiple links and subnets but never on the public internet.

Is the Global ID generation deterministic? No — that's deliberate. Two runs of this tool produce different /48s; the goal is uniqueness, not reproducibility. If you need to regenerate the same prefix, save the output.