using ULA for 'hidden' v6 devices?

Mark Andrews marka at
Wed Jan 25 22:51:13 UTC 2012

In message <Pine.LNX.4.64.1201251037480.16219 at>, "Justin M
. Streiner" writes:
> Is anyone using ULA (RFC 4193) address space for v6 infrastructure that 
> does not need to be exposed to the outside world?  I understand the 
> concept of having fc00::/8 being doled out by the RIRs never went 
> anywhere, and using space out of fd00::/8 can be a bit of a crap-shoot 
> because of the likelihood of many organizations that do so not following 
> the algorithm for picking a /48 that is outlined in the RFC.
> There would appear to be reasonable arguments for and against using ULA. 
> I'm just curious about what people are doing in practice.
> jms

A lot has to do with whether you have PA addresses of not.  As for picking
a random prefix I suspect most home CPE devices will do the right thing.
It's also easy to do the right thing.  I just did

	"dd if=/dev/random count=1 bs=5 | od -x"

and pulled the hex dig digits out to construct the ULA I use at home.  A
little bit prettier version is below.

	dd bs=5 count=1 if=/dev/random 2> /dev/null |
	od -t x1 |
	awk 'NF == 6 { print "f8" $2 ":" $3 $4 ":" $5 $6 }'

	If you don't want to use /dev/random

	(ifconfig -a ; date ; netstat -na) | md5 |
	sed 's/\(..\)\(....\)\(....\).*/f8\1:\2:\3/'

	There are lots of ways to generate a suitable prefix.

Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at

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