BIVBlog #5: To ULA or not to ULA

A sometimes rather emotionally discussed topic related to IPv6 address plans is wether to use Unique-Local addresses (ULAs) or not. As usual, the only “one size fits all” answer to that question is “it depends”; here are some of the criteria I found relevant.

Table of Contents

00:00:48 Properties of ULAs
00:01:00 ULAs behind firewalls
00:02:12 ULAs vs. ISP-requested renumberings
00:02:37 ULAs as fallback
00:03:15 Broken address selection algorithms
00:04:15 Deployment vs. renumbering problems
00:04:50 ULAs (and multiple global prefixes) in an address plan


RFC 4193: Unique Local IPv6 Unicast Addresses defines the ULA address ranges, its uses and the rationale behind it.
RFC 6724: Default Address Selection for Internet Protocol Version 6 (IPv6) specifies the address selection algorithms both for source and destination address selection.
RFC 3484: Default Address Selection for Internet Protocol version 6 (IPv6) is the predecessor to RFC 6724; some implementations are older than RFC 6724, so sometimes it helps to keep this one around, too.


Long term IPv6 evangelist/book author/trainer/consultant and generic Unix guy (*BSD, Linux, Solaris, and about a dozen more).

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