To make clear that the total communications facility may consist of multiple networks, the constituent networks are usually referred to as subnetworks. This protocol enables the host to send data across the subnetwork to another host or, if the target host is on another subnetwork, to a router that will forward the data. IP is implemented in all of the end systems and the routers. It acts as a relay to move a block of data from one host, through one or more routers, to another host.
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TCP provides a reliable connection for the transfer of data between applications : A logical connection refers to a given pair of port values. In addition to TCP, there is one other transport-level protocol that is in common use as part of the TCP/IP protocol suite: the User Datagram Protocol (UDP). UDP does not guarantee delivery, preservation of sequence, or protection against duplication.
UDP enables a procedure to send messages to other procedures with a minimum of protocol mechanism : for example, SNMP (Simple Network Management Protocol).
IP and IPv6