# IPv6 - ULAs - Am I understanding use correctly?

Brian Chandler 07/11/2018 at 16:46. 1 answers, 97 views

Tinkering more with IPv6 and trying to unlearn the IPv4 stuff. Assume my router is correctly respecting the reserved addresses.

• LLIPs aren't going to pass through to the router's outside interface and will be used only between devices connected to that router on the inside interface(s).
• GIPs are accessible from the internet/routable.
• ULAs would be used like if you had several routers (like one per room in a college) or if you were connecting sites over a VPN tunnel. So routable, but shouldn't route publicly.

Is my understanding of the ULAs use correct?

Ron Maupin 07/11/2018 at 17:39.
• 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 Architecture)
• 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 (fc00::/8) 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)

1 .Introduction

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 boundaries.

• 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 scoped addresses.

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.