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Network layer

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Third layer of the OSI model

"Layer 3" redirects here. For the MPEG-1 Audio format, seeMP3. For the layer in the cerebral cortex, seeCerebral cortex § Layered_structure.

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OSI model bylayer
7. Application layer NNTP SIP SSI DNS FTP Gopher HTTP (HTTP/3) NFS NTP SMPP SSH SMTP SNMP Telnet DHCP NETCONF more....
6. Presentation layer MIME XDR ASN.1 ASCII TLS PGP
5. Session layer Named pipe NetBIOS SAP PPTP RTP SOCKS X.225^[1]
4. Transport layer TCP UDP SCTP DCCP QUIC SPX
3. Network layer IP IPv4 IPv6 ICMP (ICMPv6) IPsec IGMP IPX IS-IS AppleTalk X.25 PLP
2. Data link layer ATM ARP SDLC HDLC CSLIP SLIP GFP PLIP IEEE 802 LLC MAC L2TP Frame Relay ITU-T G.hn DLL PPP X.25 LAPB Q.922 LAPF
1. Physical layer RS-232 RS-449 ITU-T V-Series I.430 I.431 PDH SONET/SDH PON OTN DSL IEEE 802 IEEE 1394 ITU-T G.hn PHY USB Bluetooth X.21
v t e

In the seven-layerOSI model ofcomputer networking, thenetwork layer islayer 3. The network layer is responsible forpacket forwarding includingrouting through intermediaterouters.^[2]

Functions

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The network layer provides the means of transferring variable-lengthnetwork packets from a source to a destinationhost 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 thetransport layer and issues service requests to thedata 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 asIP addresses (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 orrouters, to forward packets between networks.

Relation to TCP/IP model

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TheTCP/IP model describes the protocols used by the Internet.^[3] The TCP/IP model has a layer called theInternet layer, located above thelink 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]} 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]}

Fragmentation of Internet Protocol packets

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The network layer is responsible forfragmentation and reassembly forIPv4 packets that are larger than the smallestMTU of all the intermediate links on the packet's path to its destination. It is the function ofrouters to fragment packets if needed, and ofhosts 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 avoidpacket loss. For this,Path MTU discovery is used between endpoints, which makes it part of theTransport layer, instead of this layer.