What is Default Gateway?

Default Gateway

The method that a host uses to send messages to a destination on a remote network differs from the way a host sends messages on the same local network. When a host needs to send a message to another host located on the same network, it will forward the message directly. A host will use ARP to discover the MAC address of the destination host. It includes the destination IP address within the packet and encapsulates the packet into a frame containing the MAC address of the destination and forwards it out.

On the other hand, when a host needs to send a message to a remote network, it must use the router. The host includes the IP address of the destination host within the packet just like before. However, when it encapsulates the packet into a frame, it uses the MAC address of the router as the destination for the frame. In this way, the router will receive and accept the frame based on the MAC address.

How does the source host determine the MAC address of the router? A host is given the IP address of the router through the default gateway address configured in its TCP/IP settings. The default gateway address is the address of the router interface connected to the same local network as the source host. All hosts on the local network use the default gateway address to send messages to the router. Once the host knows the default gateway IP address, it can use ARP to determine the MAC address. The MAC address of the router is then placed in the frame, destined for another network.It is important that the correct default gateway be configured on each host on the local network. If no default gateway is configured in the host TCP/IP settings, or if the wrong default gateway is specified, messages addressed to hosts on remote networks cannot be delivered.