hop metric of `centrality'


shortest path distance on IP graph
find maximum distance from any given node to all core
center of the core: those with min maximum distance
e.g., take top 50 by this metric

top 3 prefixes
          Prefix          Outdegree
          157.130.0.0/16  1172
          144.232.0.0/16    757
          4.0.0.0/8              655