The Internet technology is connectionless. This means that there is no end-to-end setup for a session; each packet is independently routed to its destination. When a packet is ready, the host computer sends it on to another computer, known as a router. The router examines the destination address in the header and passes the packet along to another router, chosen by a route-finding algorithm. A packet may go through 30 or more routers in its travels from one host computer to another. Because routes are dynamically updated, it is possible for different packets from a single session to take different routes to the destination.
Along the way packets may be broken up into smaller packets, or reassembled into bigger ones. When the packets reach their final destination, they are reassembled at the host computer. The instructions for doing this reassembly are part of the TCP/IP protocol.
Some packet-switching networks are ``connection-oriented'' (notably, X.25 networks, such as Tymnet and frame-relay networks). In such a network a connection is set up before transmission begins, just as in a circuit-switched network. A fixed route is defined, and information necessary to match packets to their session and defined route is stored in memory tables in the routers. Thus, connectionless networks economize on router memory and connection set-up time, while connection-oriented networks economize on routing calculations (which have to be redone for every packet in a connectionless network).