indices and cidr/inet type

in looking over the section of the users manual on indices, i see that
R-Tree indices are recommended for <<, but this advice is clearly tied to
the geometric interpretation of << ("to the left of") rather than the
network interpretation ("is contained in").

what type of index is recommended when the dominant lookup on a field is

'<networkaddress>' << ipblock

where the ipblock column is of type cidr?

thanks,
  richard
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Richard Welty                                         rwelty@averillpark.net
Averill Park Networking                                         518-573-7592
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