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?


1 Answer 1

  • 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.

  • Thanks Ron. That clears things up for me. I'm starting to visualize my tinker network more like logical nets layered on top of one another. Then the main focus is the firewalls for security (I'm assuming at least per site and per node) and then routers strictly to get packets places. NAT's not a router anyway and isn't needed now. For GIP could rotate them manually for privacy on laptops from a consumer perspective, but can have different services not only on own port, but own GIP as well. This is cool. :) Jul 11, 2018 at 18:48
  • The default for most OSes is to use random addressing with privacy extensions, and the addresses will periodically change. You must override that behavior for your servers.
    – Ron Maupin
    Jul 11, 2018 at 18:52
  • Do things like SMB discovery etc. work over different router when using ULA, e.g. like if they were in the same subnet?
    – TJJ
    Nov 19, 2019 at 9:14
  • Probably not (protocols above OSI layer-4 are off-topic here, but can be asked about on Server Fault for a business network). Remember, routers route traffic between different networks, so any protocol that must operate on the same network (link-local protocols) cannot cross a router because routers strip off the LAN frame and route the packets to other networks, creating new LAN frames for the new network that have no information about the originating LAN, e.g. MAC addressing of the original LAN is gone, and MAC addressing is put on for the new LAN.
    – Ron Maupin
    Nov 19, 2019 at 14:39

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