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A default gateway is the router to which your computer addresses packet information for data destined for a computer on another network.

If you have a set of computers that can talk to each other directly, you have a network. For computers on your network to talk to computers on another network, you need a default gateway. The default gateway is a computer to which other local computers send data that is destined for a non-local computer. On computers running IP, a default gateway address is needed to reach all computers that are not on the same local IP subnet. Traditionally, the network administrator configures a computer with routing capability with first address in a range of IP addresses as the default gateway and points all clients at it.

When computers attempt to communicate with other computers that are not local to the IP address range they belong to (the subnet they belong to), the local computers must send their data to the default gateway to be forwarded. This occurs as follows:

  1. Is the remote computer's IP address on the same subnet?
  2. YES. Use ARP to get the remote computer's physical address (MAC address).
  3. NO. Use ARP to get the default gateway's physical address (MAC address).
  4. Construct a physical frame containing:
    1. The physical frame will be addressed to the physical address gathered in either step 2 or 3.
    2. The payload of the physical/data link layer frame will contain the IP header which will contain the remote computer's IP address.
  5. Transmit the frame
  6. If the frame is addressed to the default gateway, the gateway will receive the packet, find a route that matches the remote computer's IP address.

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