IP uses the default gateway address when it cannot find the destination host on the locat subnet. This is usually the router interface.
In TCP/IP, the intermediate network device on the local network that has knowledge of the network IDs of the other networks in the Internet, so it can forward the packets to other gateways until they are delivered to the one connected to the specified destination.
A default gateway refers to a device which converts data and files between two systems that use different OS/platforms or communications protocols so that the data or files can be used on either system. This is achieved when each system recognizes the other system's all seven layers of data communications model (standardized by ISO); this way differences in platforms and communications methods are detected and converted as required, enabling mutual recognition of the data or files. When connecting to the Internet, the first gateway recognized by a client PC on the network is usually regarded as the "default gateway". Or the user may designate a specific gateway as the default gateway. The user need not worry about the default gateway as long as the connection to the Internet is established via a public line using an ISP.