1. A method of generating a complex node representation of a peer group of nodes in a PNNI network system, wherein the peer group includes a plurality of border nodes, and a set of restrictive costs, defining the restrictive costs of respective paths between pairs of the border nodes, is stored in said system, the complex node representation being a representation of the peer group in which at least some of the border nodes are connected to a nucleus by respective spokes and a connection between a pair of the border nodes is represented by an exception bypass, the method comprising: (a) for each cost value C of the restrictive costs in said set, defining one or more groups of said border nodes such that, for any arrangement of the border nodes in a said group, the restrictive cost of a set of said paths which connects all the nodes in the group along a line is equal to C; (b) selecting said groups in order of cost value C; (c) for each group selected in step (b), generating a complex node representation of the group by: (c1) selecting a subset of the nodes within the group, said subset comprising either a said group for; a preceding cost value C in said order which is not contained within another said group for a preceding cost value in said order, or a single node, in dependence on the contents of the currently-selected group; (c2) for each of any said groups for a preceding cost value, which are not contained within another said group for a preceding cost value, among the remaining nodes outside the subset in the currently-selected group, defining a spoke of cost C for at least one node and defining a spoke of cost V&gE;C for each other node, and for each of any single nodes, not forming part of a said group for a preceding cost value, among the remaining nodes outside the subset in the currently-selected group, defining a spoke of cost C; (c3) for each of any said groups for a preceding cost value, which are not contained within another said group for a preceding cost value, among the remaining nodes outside the subset in the currently-selected group, defining one or more exception bypasses connecting the nodes in the group to form a spanning tree, each exception bypass having a cost equal to the restrictive cost of the path between the pair of border nodes connected thereby; and (c4) incorporating the complex node representation of said subset of nodes, wherein, if said subset comprises a single node, the complex node representation of said subset comprises a spoke of cost W&lE;C; and (d) storing the complex node representation for the last group selected in step (b) as the complex node representation for the peer group.