Within Internet data is broken into small packets called fragments. For example, say that you send a long mail on the other side of the country. TCP will divide the message int o packets. Each packet is marked with a sequence number. Data are carried out in the form of packets which contain source and destination IP addresses.
Internet routers maintain a view of the network topology in the form of routing tables.
These tables are consulted when making a packet routing decisions. The process of routing involves inspecting the destinati on address contained in the packet and based on the contents of the routing table, determining the next hop router to which the packet should be relay ed. Each router along the source and the destination nodes repeat this process until the packet is finally delievered to the destination host. Under the current addressing scheme the routers all over the world only need to maintain the network portion of the IP address. Which means that Internet address is only meaningful as long as the host using it remains connected to that network denoted by network-ID of that address.