1. ADDRESSING the Layer
   1. ELEMENTS
      1. Local Network Address
         1. category
            1. publicly concerned (TCP/IP)
            2. private
            3. IPX/SPX, SNA…
            4. unconnected TCP/IP networks
      2. Service Address
         1. inside Local Network Address
         2. identify numbers of targeted machines
            1. socket
            2. port numbers
            3. many purposes
            4. http: 80
            5. https: 8080
            6. ftp: 21
            7. sftp: 22
            8. private printing: 35
            9. telnet: 23
            10. smtp: 25
            11. DNS: 53
            12. ...
   2. Network address + Service Address ==> uniquely identify the connection
2. SWITCHING
   1. def
      1. the way packets are routed
         1. within connected network
   2. type
      1. CIRCUIT switching
         1. maintain connection during the session
      2. MESSAGE switching
         1. one hop at a time
      3. PACKET switching
         1. data packets sent thru multiple routes
         2. at destination
            1. assembled
            2. checked
            3. collected
            4. read
3. ROUTERS
   1. purpose
      1. the mechanism used to determine THE PATH
         1. connect 2 networks
         2. via a series of intermediate networks
      2. maintain connectivity among adjacent networks
   2. ROUTE TABLE
      1. def
         1. determine the path, used to connect 2 networks
      2. store
         1. in
            1. router
            2. networked computer
         2. what?
            1. the topology of the network
            2. the routes to particular network destinations
      3. MAINTAIN "table" identifies networks it attaches to availability at an interface
      4. router share with others abt networks it knows
         1. <=
            1. they build up a 'route' to connect to a certain networks
      5. HOPS
         1. distances to connect routers
   3. ROUTE DISCOVERY
      1. Number of hops = distance vector
      2. discover networks by exchanging router tables
   4. ROUTE SELECTION
      1. dynamic
         1. when a decision about the route is made at each connection
      2. static
         1. the preferred route for all communications between 2 networks
      3. If one router goes down then all interconnected networks become unavailable
4. GATEWAY SERVICES
   1. Network Layer translation
   2. relay packets thru different networks
5. CONNECTION SERVICE
   1. Network-Layer flow control
   2. Error control
   3. Packet sequence control