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==Link layer==
The protocols of the link layer operate within the scope of the local network connection to which a host is attached. This regime is called the ''link'' in TCP/IP parlance and is the lowest component layer of the suite. The link includes all hosts accessible without traversing a router. The size of the link is therefore determined by the networking hardware design. In principle, TCP/IP is designed to be hardware independent and may be implemented on top of virtually any link-layer technology. This includes not only hardware implementations but also virtual link layers such as [[virtual private network]]s and [[tunneling protocol|networking tunnels]].
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==Internet layer==
[[Internetworking]] requires sending data from the source network to the destination network. This process is called [[routing]] and is supported by host addressing and identification using the hierarchical [[IP address]]ing system. The internet layer provides an unreliable datagram transmission facility between hosts located on potentially different IP networks by forwarding datagrams to an appropriate next-hop router for further relaying to its destination. The internet layer has the responsibility of sending packets across potentially multiple networks. With this functionality, the internet layer makes possible internetworking, the interworking of different IP networks, and it essentially establishes the Internet.
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==Transport layer==
The transport layer establishes basic data channels that applications use for task-specific data exchange. The layer establishes host-to-host connectivity in the form of end-to-end message transfer services that are independent of the underlying network and independent of the structure of user data and the logistics of exchanging information. Connectivity at the transport layer can be categorized as either [[connection-oriented]], implemented in TCP, or [[connectionless]], implemented in UDP. The protocols in this layer may provide [[error control]], [[Network segmentation|segmentation]], [[Flow control (data)|flow control]], [[Network congestion|congestion control]], and application addressing ([[port numbers]]).
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==Application layer==
The application layer includes the protocols used by most applications for providing user services or exchanging application data over the network connections established by the lower-level protocols. This may include some basic network support services such as [[routing protocol]]s and host configuration. Examples of application layer protocols include the [[Hypertext Transfer Protocol]] (HTTP), the [[File Transfer Protocol]] (FTP), the [[Simple Mail Transfer Protocol]] (SMTP), and the [[Dynamic Host Configuration Protocol]] (DHCP).<ref name="RxqD0">{{cite book|url=http://www.kohala.com/start/tcpipiv1.html|title=TCP/IP Illustrated: the protocols|isbn=0-201-63346-9|first=W. Richard|last=Stevens|author-link=W. Richard Stevens|date=February 1994|publisher=Addison-Wesley |access-date=April 25, 2012|archive-date=April 22, 2012|archive-url=https://web.archive.org/web/20120422024917/http://www.kohala.com/start/tcpipiv1.html|url-status=live}}</ref> Data coded according to application layer protocols are [[encapsulation (networking)|encapsulated]] into transport layer protocol units (such as TCP streams or UDP datagrams), which in turn use [[lower layer protocol]]s to effect actual data transfer.
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