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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For decades, the keystone of the TCP/IP protocol architecture has been IP.
The header, together with the segment from the transport layer, forms an IP-level PDU referred to as an IP datagram or an IP packet.The header includes 32-bit source and destination addresses.
In 1995, the Internet Engineering Task Force (IETF), which develops protocol standards for the Internet, issued a specification for a next-generation IP, known then as IPng. This specification was turned into a standard in 1996 known as IPv6.
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DS : Differentiated services field
ECN : Explicit congestion notification field
TCP/IP Applications
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.
FTP
TELNET
etc.