Editing Network layer

{{Short description|Third layer of the OSI model}}
{{redirect|Layer 3|the MPEG-1 Audio format|MP3|the layer in the cerebral cortex|Cerebral cortex#Layered_structure}}
{{more footnotes|date=October 2009}}
{{OSIModel}}
In the seven-layer [[OSI model]] of [[computer networking]], the '''network layer''' is '''layer 3'''. The network layer is responsible for [[packet forwarding]] including [[routing]] through intermediate [[Router (computing)|routers]].<ref>{{cite web|url=http://searchunifiedcommunications.techtarget.com/definition/layer-3 |title=Layer 3 |publisher=techtarget.com |access-date=2017-05-11}}</ref>

==Functions==
The network layer provides the means of transferring variable-length [[network packet]]s from a source to a destination [[Host (network)|host]] via one or more networks. Within the service layering semantics of the OSI (Open Systems Interconnection) network architecture, the network layer responds to service requests from the [[transport layer]] and issues service requests to the [[data link layer]].

Functions of the network layer include:
; [[Connectionless communication]]
: For example, [[Internet Protocol]] is connectionless, in that a data packet can travel from a sender to a recipient without the recipient having to send an acknowledgement. Connection-oriented protocols exist at other, higher layers of the OSI model.
; Host addressing
:Every host in the network must have a unique address that determines where it is. This address is normally assigned from a hierarchical system.  For example, you can be:
:
:"Fred Murphy"                                                    to people in your house, 
:"Fred Murphy, 1 Main Street"                             to Dubliners,
:"Fred Murphy, 1 Main Street, Dublin"                 to people in Ireland, 
:"Fred Murphy, 1 Main Street, Dublin, Ireland"    to people anywhere in the world. 
:
:On the Internet, addresses are known as [[IP address]]es (Internet Protocol).
:
; Message forwarding
:Since many networks are partitioned into subnetworks and connect to other networks for wide-area communications, networks use specialized hosts, called gateways or [[Router (computing)|routers]], to forward packets between networks.

==Relation to TCP/IP model==
The [[TCP/IP model]]  describes the protocols used by the Internet.{{Ref RFC|1122}} The TCP/IP model has a layer called the [[Internet layer]], located above the [[link layer]]. In many textbooks and other secondary references, the TCP/IP Internet layer is equated with the OSI network layer. However, this comparison is misleading, as the allowed characteristics of protocols (e.g., whether they are connection-oriented or connection-less) placed into these layers are different in the two models.{{citation needed|date=November 2016}} The TCP/IP Internet layer is in fact only a subset of functionality of the network layer. It describes only one type of network architecture, the Internet.{{citation needed|date=November 2016}}

==Fragmentation of Internet Protocol packets==
The network layer is responsible for [[IP fragmentation|fragmentation and reassembly]] for [[IPv4]] packets that are larger than the smallest [[Maximum Transmission Unit|MTU]] of all the intermediate links on the packet's path to its destination. It is the function of [[Router (computing)|router]]s to fragment packets if needed, and of [[Host (network)|host]]s to reassemble them if received.

Conversely, [[IPv6]] packets are not fragmented during forwarding, but the MTU supported by a specific path must still be established, to avoid [[packet loss]]. For this, [[Path MTU discovery]] is used between endpoints, which makes it part of the [[Transport layer]], instead of this layer.

==Protocols==
The following are examples of protocols operating at the network layer.
{{div col|colwidth=30em}}
* CLNS, [[Connectionless-mode Network Service]]
* DDP, [[Datagram Delivery Protocol]]
* EGP, [[Exterior Gateway Protocol]]
* EIGRP, [[Enhanced Interior Gateway Routing Protocol]]
* ICMP, [[Internet Control Message Protocol]]
* IGMP, [[Internet Group Management Protocol]]
* IPsec, [[IPsec|Internet Protocol Security]]
* IPv4/IPv6, [[Internet Protocol]]
* IPX, [[Internetwork Packet Exchange]]
* LLARP, [[Low Latency Anonymous Routing Protocol]] 
* OSPF, [[Open Shortest Path First]]
* PIM, [[Protocol Independent Multicast]]
* RIP, [[Routing Information Protocol]]
{{div col end}}

==References==
{{Reflist}}
* {{cite book |first=Andrew S. |last=Tanenbaum |title=Computer networks |publisher=[[Prentice Hall]] |location=[[Upper Saddle River, New Jersey]] |year=2003 |isbn=0-13-066102-3 |url-access=registration |url=https://archive.org/details/computernetworks00tane_2 }}

==External links==
* [http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.136.9497&rep=rep1&type=pdf OSI Reference Model—The ISO Model of Architecture for Open Systems Interconnection], Hubert Zimmermann, IEEE Transactions on Communications, vol. 28, no. 4, April 1980, pp.&nbsp;425 – 432. (PDF-Datei; 776&nbsp;kB)

[[Category:OSI model]]

[[de:OSI-Modell#Schicht 3 – Vermittlungsschicht]]