Routers

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The devices discussed so far enable physical links where the only type of addressing used identifies a host hardware interface:

• Ethernet switches and Wi-Fi access points forward frames using MAC addresses
• network segment is where hosts can send frames to one another using their MAC addresses
• Digital modems, ONTs, and cellular radios transmit data over DSL, cable, fiber, satellite, and cellular links to connect a local network or device to an ISP
  • is typically a point-to-point link and so does not require unique interface addressing
• These network segments use different media types and have no physical or logical means of communicating with one another
  • When you want to connect a local network to the internet:
    • need to use a protocol that can distinguish between the private LAN and public WAN
    • an intermediate system with interfaces in both networks
    • protocol used to implement this is the Internet Protocol (IP), and the intermediate system is a router
• a switch forwards frames using MAC (hardware) addresses
• a router forwards packets through the internet using IP addresses
• MAC address only identifies a hardware port
• IP address contains the identity of both the network and a single host within that network
• several types of routers and different uses for them:
  • SOHO router often simply routes between its local network interface and its WAN/Internet interface
  • enterprise network is likely to use different router models to perform different routing tasks:
    • LAN router
      • divides a single physical network into multiple logical subnetworks
      • Each logical network becomes a separate broadcast domain
      • having too many hosts in the same broadcast domain reduces performance
      • also a security benefit because traffic passing from one logical network to another can be subject to filtering rules
      • generally has only Ethernet interfaces
    • WAN or border router
      • forwards traffic to and from the Internet or over a private WAN link
      • has an Ethernet interface for the local network and a digital modem interface for the WAN