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What is a default route and when is it used?

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A default route, also known as the default gateway or default route gateway, is a routing term used in computer networking to represent the route that a packet takes when there is no specific entry in the routing table for the destination IP address of that packet.

Here is a more technical breakdown:

  1. Routing Table:
    • In networking, devices such as routers maintain a routing table. This table contains information about how to reach different networks or hosts based on their IP addresses.
    • Each entry in the routing table typically includes a destination IP address and the corresponding next-hop address (the next device the packet should be sent to in order to reach the destination).
  2. Default Route Entry:
    • The default route is a special entry in the routing table that serves as a catch-all for packets with destination IP addresses that do not match any specific entry in the table.
    • The default route is denoted by the destination IP address 0.0.0.0 (or sometimes by the network address 0.0.0.0 with a subnet mask of 0.0.0.0).
  3. When is it Used:
    • When a network device receives a packet and looks up the destination IP address in its routing table, it checks for a specific entry for that address.
    • If a specific entry is found, the packet is forwarded based on the information in that entry.
    • If no specific entry is found (i.e., the destination IP address does not match any entry), the device uses the default route to forward the packet.
    • This is particularly useful when a device needs to send a packet to a destination outside of its directly connected networks.
  4. Configuration:
    • The default route is typically configured on routers or gateways by network administrators.
    • On a router, the default route is usually set to the IP address of the next-hop router or gateway that knows how to reach external networks (such as the internet).
  5. Example:
    • If a computer wants to send a packet to a server on the internet, and it doesn't have a specific route for that server in its routing table, it will use the default route to reach the gateway router, which in turn will route the packet towards the internet.