Learn in the textbookConnecting Devices That Make Up a Network
DefinitionA default gateway (デフォルトゲートウェイ) is the role of the exit device a host first hands a packet to when the destination lies outside its own network, and a router usually fills it.
ExplanationAlong with its own IP address and other settings, each device is configured with the IP address of its default gateway. Even without detailed route information for each destination, the device can hand outbound traffic to this exit and let it be relayed onward from there.
AnalogyMail for anywhere outside the house only needs to go in the nearest postbox. The post office carries it the rest of the way.
Memory hookDefault means the value used when nothing else is specified. It is the standard gate that traffic takes when no specific path has been set for its destination.