An internet is a combination of networks connected by（）. When a datagram goes from a source to a（）， it will probably pass many（）until it reaches the router attached to the destination network. A router receives a（）from a network and passes it to another network. A router is usually attached to several networks. When it receives a packet， to which network should it pass the packet? The decision is based on optimization: Which of the available（）is the optimum pathway?