Network hardware
•transmission technology
•broadcast networks:single communication channel sharedby all network nodes
•can send to: one node, all nodes, (maybe) group of nodes
•address field in message specifies receiver(s)
•point-to-point networks:many possible connection paths between any pair of nodes
•message may have to pass through intermediate nodes on the way from sender to receiver
•usually, need a routing algorithmto decide ifa path exists from sender to receiver, and --if multiple such paths exist --which one(s)to use