After the host name or NetBIOS name is resolved to an IP address, the IP packet must be sent by the sending host to the resolved IP address. Routing is the process of forwarding a packet based on the destination IP address. Routing occurs at a sending TCP/IP host and at an IP router. A router is a device that forwards the packets from one network to another. Routers are also commonly referred to as gateways . In both cases, sending host and router, a decision has to be made about where the packet is forwarded.
To make these decisions, the IP layer consults a routing table stored in memory. Routing table entries are created by default when TCP/IP initializes and additional entries are added either manually by a system administrator or automatically through communication with routers.