(第1空)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?