We have reached the top of our OSI model:
Applications are probably the topic that you can relate most easily, as everyone uses applications all the time: WhatsApp, Gmail, Firefox, Chrome, Candy Crush, etc.
Now we will discuss how do these applications work.
For the other layers we had dependencies: most of the time we were discussing ‘you need this…’ and now we are working on top of it, and going to the beginning of the layer to connect things together. The Application Layer is a bit different, in the sense that we have independent applications, with way less dependencies than before. It is mainly how some type of application works. With the exception of DNS, used by all applications (and what we will discuss first).
Machines on the Internet are identified by their IP Address. We already mentioned this in previous layers. However, these addresses are difficult for humans to remember: how will remember something like $\text{http://4.31.198.44/rfc/rfc1035.txt}$, or $\text{[email protected]}$? Moreover, IP address change over time, so you would have to be changing these address all the time.
As a solution we have the Domain Name System (DNS), which translates human readable names into IP addresses. This can also deal with the problem that IPs dynamically change.
All applications use DNS.
How does these human readable names look like? This should be pretty unsurprising as all people already know how URLs look like. But let’s see it nevertheless.
DNS name space follows a hierarchical structure.
Top level domains are controlled by Internet Corporation for Assigned Names and Numbers (ICANN). They are located in the US.
‘.edu’ and ‘.gov’ are generic domains, but mostly used by organizations in the United States.
Organizations can request second-level domains from registrars.
Countries have their own first-level domains. Countries can also decide about their lower-level domains.
If you control a domain, you can specify arbitrary subdomains.
United Kingdom, for example, uses $\text{ac.uk}$ for academic use and $\text{co.uk}$ for commercial use.