See http://code.google.com/p/ipaddr-py/
With this, we can do
py> p=ipaddr.IPv6("2001:888:2000:d::a2")
py> p.SetPrefix(64)
py> p
IPv6('2001:888:2000:d::a2/64')
py> p.network_ext
'2001:888:2000:d::'
Posted inprograming python
Extract ipv6 prefix in python
