>> Hello. We really have
to appreciate what DNS does for us because
people are generally good with remembering
names but not with remembering IP addresses.
Most of us like a user-friendly name.
For example, I know how to go to Amazon.com,
but I have no clue what the IP address is for it.
But we have to remember that when
hosts are making a network connection,
one of the things they need is an IP address.
That's a layer 3 resolution that has to happen.
We go to DNS for that.
Our clients are configured to know who
their local DNS server is.
If there's any naming that's used for connection,
the client immediately carries their local DNS server.
As long as that local DNS server
has an information stored,
then it responds with an IP address.
The client is then able to add that
in the layer 3 packet header,
and we can make that connection.
That's assuming the local DNS
>> server has an information.
>> In a moment, we'll talk about what happens
when it does not have an information.
With DNS, the information is stored in a database,
and today it's a dynamic database,
where hosts are able to update DNS
perhaps if their name changes
or their IP address changes.
As new hosts come onto the network,
they're able to register with DNS.
DNS is a lot easier to manage than it was
years ago when we had
a static host file that included this information.
As DNS has evolved and started
allowing hosts to register dynamically,
it's even easier than it used to be.
But it's a really critical service,
and everything is based on the database,
and the database contains a series of records.
Take note of these DNS record types
as they come up multiple times on the exam.
The idea is for specific queries,
specific records are used.
The main one that we think about as
an A record or a host record.
This is the record that takes
a known fully qualified domain name,
FQDN, and resolves it to an IP address.
We connect it to Amazon.com and we
get an IP address that matches the server's name.
We can do that for local usage.
Or, as long as we're configured
>> to the Internet servers,
>> we can get name resolution to
any server anywhere in the world.
That's a host record.
We also have an AAAA record or a quad A record.
It's designed to provide the resolution of
an FQDN to IP version 6 addresses.
Then the PTR or
Pointer Record is actually the opposite
>> of a host record.
>> If you have an IP address,
it will find the FQDN it's associated with.
This is something that is used by a lot of tools in
the network and anti-malware software applications
that use IP addresses.
This record allows them to do a reverse lookup on
the IP address and find the FQDN for it.
As a matter of fact, they call it a
>> reverse lookup zone,
>> whereas when you're using a host record,
that's a forward lookup.
Now, there are a handful of others.
The start of authority or
SOA record is the name server
that is authoritative for the zone.
Your start of authority is
the first DNS server that
has a name resolution for that zone.
We may have other name servers for redundancy.
That's what the NS records are for.
NS records list name service for the zone.
The SRV or service records are really important because
they list out the critical network services
and their IP addresses.
For instance, if a host needs to know where
a domain controller is, it asks DNS.
The DNS uses the service record
and provides the IP address for it.
The service records have
all the services and which servers are running them.
There's also a record type called the
CNAME or ALIAS record.
If you want clients to connect to
a specific co-host and you need it to have
a professional name but on the back-end you
want to name it after some Star Trek
character for the fun of it,
you can create an ALIAS for it.
The professional sounding web address
will point to your Star Trek name server,
or there may be other times where you need to use
an ALIAS to point out
a new name to an old name, for example.
The last type of record is an MX record.
This is going to indicate where the mail servers are.
These records are really the heart and soul of DNS.
These are the records that are used anytime
you make a query to the DNS server.
Now, the way DNS works is
that it uses a hierarchical database.
For example, let's see that I'm trying
to get to www.google.com.
When I type that into my browser,
I'm sending a DNS query to my local DNS server.
It's called a DNS resolver.
Now, because google.com domain is
not managed by my local DNS server,
my DNS server sends out
some queries to learn where
the server is I'm looking for.
The first thing my DNS server
does is contact of root server.
There are multiple root servers on the Internet.
The root server is like the ultimate point
of origin for DNS.
The DNS server says, hey,
root, do you happen to know where google.com is?
The root says, no,
but I do know where the.com server is.
My DNS server goes to
the top-level domain,.com in this case,
and asks where the google.com is.
The.com server says,
no, but I do know where the Google name server is.
Then my DNS server sends a query to
the Google name server and
asks if it knows where google.com.
The Google name servers says yes,
and here is the IP address for our web server.
My local DNS server passes it back to me as the client,
and now I'm able to connect to
the Google's web server by IP.
Now, there are other ways to
>> expedite this, like caching.
>> But just in and of itself,
this is the hierarchy that DNS
uses to provide name resolution.
Don't forget, the DNS is critical in
a network because in addition to name resolution,
it also tracks critical services,
like where your mail services are,
and it's needed for troubleshooting utilities.
DNS is really important,
and it's a hierarchical database,
starting out with the root and the top-level domain,
and then secondary levels.