OSI Seven Layer Network Reference Model - Addressing and Example
The OSI (Open Systems Interconnection) Seven Layer Network Reference Model is the basic for learning network infrastructure. It is an abstract description of computer network protocol design.
From top to bottom, the name of the layers are Application, Presentation, Session, Transport, Network, Data-Link, and Physical Layers.
Each layer provides services to the layer above it and receives services from the layer below it. For example, network layer (layer 3) will provide service for transport layer (layer 4) and receive service from layer 2, which is the data link layer.
The original Internet TCP/IP protocol specifications defined a four layer model (application, transport, network and physical).
Example of Application layer (Layer 7) services are FTP, DNS, SNMP, HTTP and so on. Example of Presentation Layer (Layer 6) services are HTML, GIF, ASCII, EBCDIC and so on. Appletalk ASP, NetBIOS names can be classified as in the Session Layer (Layer 5). The popular examples of Layer 4 or Transport Layer are TCP and UDP. The IP or Internet Protocol is an example of Network Layer (Layer 3).
It is also interesting to study about the addressing scheme use in this network model.
When you access to a web page, you will key in the URL or the web address of the web page. This URL, for example http://asiavalueinvestor.com is refer to the application layer address.
In order for your computer to connect to the server or service, it will also needs the transport layer address, which is the port number of the web server which offer the web service. In this case, as the protocol is http, the port number used is 80.
In the case of network layer, your computer will also needs the network layer address for the web site, which is the IP address in this case.
And lastly, your computer also needs the physical address in order to successfully transfer data or request to the destination. The physical address can be the network gateway in your network or it can be the physical address of the web server, depends on how far the web server is from your computer.
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