A connectivity look up structure is maintained for a network that comprises a plurality of nodes, each node is connectable to one or more other nodes, and nodes that are connected tend to be local to one another in the network, and the number of node connections in the network tends to be sparse in relation to the number of potential node connections in the network. The connectivity look up structure stores, for a given node, an address of each other node that is connected to the given node, wherein the stored address for the other node is represented as a run-length encoded difference between a full network address of the given node and a full network address of the other node.