This attribute specifies the network number of matching networks in RIP
packets to be filtered or passed depending on the disposition of the filter.
The network number can be entered either as a dashed hex string or a
string of hex digits entered in pairs of hex digits up to a maximum
length of 8 digits; in addition to hex digits, use of the character '?' to
indicate wildcarding is allowed. (For example a valid network number
would be ff-ff-ff-?? or ffffff??). Wildcard characters may be used only in
pairs to replace a pair of hex digits. Filters with a network number
preceded by a caret (^) should be interpreted as matching all RIP packet
tuples except for the tuples with this network number. For a given filter,
only networks matching all the specified attributes of that filter are
filtered or passed depending on the disposition of the filter.