The last leg of the show I saw a model we should examine is the application layer.
There are a lot of application protocols.
As for now we will only be interested in the protocols associated with the network infrastructure services.
The first of those is DNS.
This is a protocol which serves as the Internet's telephone directory in order for you to connect to
a remote computer using its qualified name.
Your host must know its IP address there must exist a database which will translate the name you know
into the IP address.
This is performed by the DNS servers and the protocols created to enable them to communicate.
There are quite a lot of computers on the internet.
If there was only one DNS server then it would have to store a lot of information and would be permanently
blocked due to the number of requests to handle
to counter this problem.
The DNS database has a hierarchical structure at the top.
There are the root servers the root zone.
These are the servers responsible for the root domain domains such as COM or gov etc or at subdomains
each DNS server can be responsible for one or more domains.
The client side of the DNS is called the DNS resolver.
If a client wants to connect to the computer w w w dot Microsoft dot com W W W is the name of the computer.
Microsoft is the name of the subdomain and com is the domain name.
It connects to its local DNS server first.
This is very important.
Later we will find out that controlling the DNS server allows you to control the data transmitted the
host connects to the local DNS server and request for the IP address of w w w Microsoft dot com computer
the local DNS server probably will not keep the IP address as we said.
Each server is responsible for its own subdomain.
Your local DNS server will surely know the address of one of the root zone servers so it will request
one of those servers for the host address of WWE w dump Microsoft dot com.
Unfortunately the root server won't be able to answer this request either.
It will respond however with the address of the DNS server lower in the hierarchy which is responsible
for the domain com.
At this point your local DNS will connect to this server and will request the address w w w.
Microsoft dot com computer.
Your request will probably be forwarded to an authoritative server responsible for the Microsoft Dotcom's
zone.
At this point the only two responses possible are either the computer's IP address or that such as the
computer doesn't exist.
DNS servers store in their databases information not only about the hosts type of records but also about
record types as well two types will be especially interesting for us.
These are DNS records indicating other DNS servers and the MX records indicating mail servers.