OSI Layer Explained

From my experience, not many people know what OSI layer is all about. They all know vague definitions for each stage of OSI model but many IT individuals find it tough to explain it very well.

Here I will try my best to explain this model in a way that you can memorize and maybe use in future for reference.

First of all, do you know what are the seven layers in OSI model…. is it easy for you to name them out without thinking about the order? probably not or may be you just can’t remember them because you looked at them 6+ months ago.

So lets address that issue with: Please Do Not Teach Students Pointless Acronyms

Does it make sense?

Please : Physical Layer
Do : Data Link Layer
Not : Network Layer
Teach : Transport Layer
Students : Session Layer
Pointless : Presentation Layer
Acronyms : Application Layer

Yes, Ofcourse it does. That’s out of the way then.

Lets start with the Application Layer. This layer comprises of protocols that we use in our daily life. These protocols are HTTP (web browsing), SMTP (Mail Servers), FTP or SFTP (File Transfers). A typical example of this layer would be use of a web browser like chrome, as soon as you enter a URL in the browser and hit enter. Your browser initiates a get request. Now this get request and the protocol it uses is application layer. The format of the request tends to be in the Presentation Layer of the OSI model. So for a get request it will be the format of http get header.

How to check if remote port is open in linux (if nc isn’t available) ?

Its actually quite simple.

Just install tcping package. In Centos you can download it from EPEL repo:

[[email protected] ~]# yum search tcping
Loaded plugins: fastestmirror
tcping.x86_64 : Check of TCP connection to a given IP/Port
[[email protected] ~]# yum install tcping
#### Once installed #####
[[email protected] ~]# tcping localhost 80
localhost port 80 open.

Hope this helps.