●  An internet is a combination of networks connected by  （71）  . When a datagram goesfrom a source to a  （72）  , it will probably pass many  （73）    until it reaches the routerattached to the destination network. A router receives a  （74）    from a network andpasses  it  to  another  network.  A router is usually attached to several networks. When  itreceives      a      packet,      towhich  network  should  it  pass  the  packet、  The  decision  is  based  on optimization:Which of the available  （75）  is the optimum pathway、