IP Routing

Local delivery

when the IP software is presented with a datagram for delivery, it first checks the network part of the destination address to see if matches its own network number. If it does, then the datagram can be delivered locally, and is simply handed to the physical network delivery system (typically an Ethernet device driver) for direct delivery over the local network.
 

Internet delivery

if the network numbers are different, then the datagram must be delivered to a local router (or IP gateway), using direct delivery as above, for "on-delivery" across the Internet.
 

• Datagrams are forwarded from router to router across each of the intervening networks until they reach a router connected to a network where they can be locally delivered.
• The IP software in each router must keep a routing table to know the IP address of an appropriate next-hop router which brings the datagram closer to its destination. For hosts, this is commonly done by specifying a default route.
• Note that routers must have (at least) two IP addresses!