> We  didn't  see any issue in plan9. And the networking base
> all are IPv4

Plan 9 uses IPv6 format addresses by default, though there is
no IPv6 protocol stack for handling IPng traffic. There are a 
lot of hooks in place for the soul that wishes to add such 
support, it seems. It looks like such an effort would drop in rather 
nicely. See ip (2) for more info.
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pip