Network identification list. This identifies which
networks will match this rule. If non-null, The octet
string contains one or more 3-tuples of this form:
first octet: exact (1) or range (2)
next 4 octets: network number
next 4 octets: network mask
An entry with an 'exact' tag means to only match the
specific network advertisement (number & mask). An
entry with a 'range' tag means to match any network
number that falls in the range indicated by the number
and mask.
An 'exact' encoding of 0.0.0.0/0.0.0.0 means 'match the
default route. A 'range' encoding of 0.0.0.0/0.0.0.0 means
'match any route'.
A null string also means 'match any route'.