IP Routing

Local delivery
 

When the IP software is presented with a datagram for delivery, it first checks the network/subnet part of the destination address to see if matches its own network/subnet number -- the current netmask is used to discover if these match. If the network/subnet part does match, 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 sent to a directly-connected router (or IP gateway), using direct delivery as above, for "on-delivery" across the Internet. Note that when the datagram is received at the router, its destination IP address is not that of the router.
 

• 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.
• Note that routers must have (at least) two IP addresses, since they are connected to (at least) two different networks/subnets!