Network layer

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OSI Model
7	Application layer
6	Presentation layer
5	Session layer
4	Transport layer
3	Network layer
2	Data link layer LLC sublayer MAC sublayer
1	Physical layer

The network layer is level three of the seven level OSI model as well as of the five layer TCP/IP reference model. In the four layer TCP/IP reference model it is called Internet layer, which is the second layer from below. It responds to service requests from the transport layer and issues service requests to the data link layer.

In essence, the network layer is responsible for end to end (source to destination) packet delivery, whereas the data link layer is responsible for node to node (hop to hop) packet delivery.

The network layer provides the functional and procedural means of transferring variable length data sequences from a source to a destination via one or more networks while maintaining the quality of service requested by the transport layer. The Network layer performs network routing, flow control, network segmentation/desegmentation, and error control functions.

The network layer deals with transmitting information all the way from its source to its destination - transmitting from anywhere, to anywhere. Here are some things that the network layer needs to address:

Internet protocol suite

5. Application layer

DNS, TLS/SSL, TFTP, FTP, HTTP, IMAP4, IRC, MIME, POP3, SIP, SMTP, SNMP, SSH, TELNET, RTP, SDP, …

4. Transport layer

TCP, UDP, RSVP, DCCP, SCTP, …

3. Network layer

IP (IPv4, IPv6), ICMP, IGMP, ARP, RARP, …

2. Data link layer

Ethernet, Wi-Fi, PPP, FDDI, ATM, Frame Relay, GPRS, Bluetooth, …

1. Physical layer

Modems, ISDN, SONET/SDH, RS232, USB, Ethernet physical layer, Wi-Fi, GSM, Bluetooth, …

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Is the network connection-oriented or connectionless?

For example, snail mail is connectionless, because you can send a letter to someone without them doing anything and they will receive the letter. On the other hand, the telephone system is connection-oriented, because the other party is required to pick up the phone before communication can be established. The OSI Network Layer protocol can be either connection-oriented, or connectionless. The TCP/IP Internet Layer (equivalent to OSI's Network Layer) supports only the connectionless Internet Protocol (IP).

What are the Global Addresses?

Everybody in the network needs to have a unique address which determines who they are. This address will normally be hierarchical, so you can be "Fred Murphy" to Dubliners, or "Fred Murphy, Dublin" to people in Ireland, or "Fred Murphy, Dublin, Ireland" to people anywhere in the world. On the internet, these addresses are known as IP Numbers.

How do you forward a message?

This is of particular interest to mobile applications, where a user may rapidly move from place to place, and it must be arranged that his messages follow him. Version 4 of the Internet Protocol (IPv4) doesn't really allow for this, though it has been hacked somewhat since its inception. Fortunately, the forthcoming IPv6 has a much better designed solution, which should make this type of application much smoother.

In the traditional postage system (commonly referred to as snail mail) this role is provided by the postman (to some extent).