Seven Layer Model

The OSI Seven Layer Model

In the early days of computer networking (pre 1969), connections between were hardwired, and the software to implement the connection was custom tailored to the operating systems of the connected computers. As the ARPANET (predecessor of the modern Internet) was being built, it became clear that connecting a large number of computers together in one large network required a well designed network architecture.

Gradually the Open Systems Institute (OSI) Seven Layer Model was developed, starting in 1977.

Level	Name	Description	Examples
1	Physical	Defines the method that the data bits are Sent over the network (electric, light, radio).	DSL^*, ISDN, RS-232
2	Data Link	Determines the low level details of how data is moved between hosts (individual computers) and/or network equipment (routers and bridges).	Ethernet 802.3^*, PPP, SLIP, HDLC
3	Network	Determines route from source host to destination host. Controls congestion.	IP^*, IPX, DDP
4	Transport	Sends data from application to application. Responsable for error detection and retransmission.	TCP^*, UDP, OSPF, BGP
5	Session	Coordinates dialogs or data exchanges between applications. Allows interrupted sessions to resume without starting over.	NetBios^*, AppleTalk, Sockets
6	Presentation	Handles encryption, compression, or other encoding of data.	ASCII^, MIME^, UTF-8, XML, SOAP, XDR
7	Application	Supports end user application software such as e-mail, file transfer, or other network software.	HTTP^, FTP^, SMTP, telnet^*

See this Wikepedia article for more details about the OSI Model.

Some Telecommunications Equipment

A hub is a repeater that unselectively forwards data at OSI Level 2 from point to point or from point to multipoint.
A switch is more selective than a hub. It looks at the address in a packet before deciding where to forward it. Traditionally, switches operate at OSI Level 2, but modern switches can also operate at Level 3.
A bridge connects different network segments. While a hub passes on information using the same protocol, a bridge can translate information from one protocol to another. A firewall is a special type of bridge that blocks potentially dangerous data from entering a network segment.
A router is more intelligent that a switch. Traditionally, routers operate at OSI Level 3, but modern routers can operate at level 2 or even level 4. The name layer 3 switch is sometimes used for a router.
The traditional distinctions between switches and routers have been blurred for more than ten years.