IP Packet Routing

When an IP packet needs to be forwarded, the router (or host) has two options. If it's meant for a device on the same network, the data is sent there using a lower-level protocol (such as Ethernet, PPP, etc.).

If a packet needs to sent to a remote network, the IP protocol stack looks up the network address of the machine in its routing table. If it appears in the table, the address of the "next hop" router is listed in the table. The packet is then sent to that router using the relevant lower-level protocol.

If the network address does not appear there, the packet is sent to the router of the 'default route'. If no such route exists, the packet is discarded.