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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A number of applications have been standardized to operate on top of TCP :
SMTP (Simple Mail Transfer Protocol) : basic electronic mail transport facility, mechanism for transferring messages among separate hosts. Once a message is created, SMTP accepts the message and makes use of TCP to send it to an SMTP module on another host.
FTP (File Transfer Protocol) : used to send files from one system to another under user command. FTP sets up a TCP connection to the target system for the exchange of control messages. This connection allows user ID and password to be transmitted and allows the user to specify the file and file actions desired.
TELNET
etc.