ipfw and ipv6 mask/masklen

[Jason Fesler]

For IPv4, I see this with ipfw:

              addr/masklen
                      Matches all addresses with base addr (specified as an IP
                      address, a network number, or a hostname) and mask width
                      of masklen bits.  As an example, 1.2.3.4/25 or 1.2.3.0/25
                      will match all IP numbers from 1.2.3.0 to 1.2.3.127 .

Does anyone have a suggestion on how to do similar for IPv6?

I want to block specific Teredo/IPv4 addresses; the IPv4 address is 
encoded at the end.  The bits in the middle are variable per connection 
(with the port number being part of the variable).

I don't want to block out Teredo entirely (or even a specific relay 
entirely), as that would be overboard for my needs. So, CIDR notation 
prefix/length matching won't work.

--
  Jason Fesler, email/jabber <jfesler at gigo.com> resume: http://jfesler.com
  "Give a man fire, and he'll be warm for a day;
  set a man on fire, and he'll be warm for the rest of his life."