- Link-local addresses (
fe80::/10) only work on the link where they
are addressed. Evey link uses the same addressing. (see RFC 4291,
IP Version 6 Addressing Architecture)
- Global addresses (
2000::/3) are globally unique (ignoring anycast
for now), and packets addressed with global addresses can be routed
over the public Internet. (see RFC 4291, IP Version 6 Addressing
- Unique-Local addresses (
fc00::/7) are blocked by ISPs, so packets
addressed with ULA cannot be routed on the public Internet, and are
for traffic that will never be sent on the public Internet.
There are two parts to ULA. The first half of the range (
is reserved for assignment by a yet-to-be-determined global
authority. The second half of the range (
fd00::/8) is for local
assignment, but the next 40 bits of the prefix must be randomly
assigned, and you cannot use consecutive 48-bit prefixes. (see RFC
4193, Unique Local IPv6 Unicast Addresses)
This document defines an IPv6 unicast address format that is
globally unique and is intended for local communications [IPV6].
These addresses are called Unique Local IPv6 Unicast Addresses and
are abbreviated in this document as Local IPv6 addresses. They are
not expected to be routable on the global Internet. They are
routable inside of a more limited area such as a site. They may
also be routed between a limited set of sites.
Local IPv6 unicast addresses have the following characteristics:
Globally unique prefix (with high probability of uniqueness).
Well-known prefix to allow for easy filtering at site
Allow sites to be combined or privately interconnected without
creating any address conflicts or requiring renumbering of
interfaces that use these prefixes.
Internet Service Provider independent and can be used for
communications inside of a site without having any permanent or
intermittent Internet connectivity.
If accidentally leaked outside of a site via routing or DNS,
there is no conflict with any other addresses.
In practice, applications may treat these addresses like global
This document defines the format of Local IPv6 addresses, how to
allocate them, and usage considerations including routing, site
border routers, DNS, application support, VPN usage, and guidelines
for how to use for local communication inside a site.
Every interface will have a link-local address. Each interface can also have multiple global and/or ULA addresses. The global addresses are used to communicate on the public Internet (or internally,too), and the ULA addresses can be used for internal site communication to prevent the traffic from ever going out on the public Internet.