[arch.py]

FIX: get_if_addr6() did return a wrong value
CHANGE: get_if_raw_addr6() bases now on get_if_addr6()

[ospf.py]
FIX: re-inserted default values of IPv6 prefixes

[isis.py]
FIX: removed unnecessary length check

[fields.py]
NEW: added some comments to the IP prefix formats
FIX: unified machine representation of IP prefixes