Ccna routing and Switching Complete Study Guide

Domain Name Service (DNS)

Domain Name Service (DNS) resolves hostnames—specifically, Internet names, such as 

www.lammle.com

. But you don’t have to actually use DNS. You just type in the IP address 

of any device you want to communicate with and find the IP address of a URL by using the 

Ping program. For example, 

>ping www.cisco.com

 will return the IP address resolved by 

DNS.

An IP address identifies hosts on a network and the Internet as well, but DNS was 

designed to make our lives easier. Think about this: What would happen if you wanted to 

move your web page to a different service provider? The IP address would change and no 

one would know what the new one is. DNS allows you to use a domain name to specify an 

IP address. You can change the IP address as often as you want and no one will know the 

difference.

To resolve a DNS address from a host, you’d typically type in the URL from your favor-

ite browser, which would hand the data to the Application layer interface to be transmitted 

on the network. The application would look up the DNS address and send a UDP request 

to your DNS server to resolve the name (Figure 3.10).

If your first DNS server doesn’t know the answer to the query, then the DNS server for-

wards a TCP request to its root DNS server. Once the query is resolved, the answer is trans-

mitted back to the originating host, which means the host can now request the information 

from the correct web server.

DNS is used to resolve a fully qualified domain name (FQDN)—for example, 

www 

.lammle.com

 or 

todd.lammle.com

. An FQDN is a hierarchy that can logically locate a  

system based on its domain identifier.

If you want to resolve the name todd, you either must type in the FQDN of 

todd 

.lammle.com

 or have a device such as a PC or router add the suffix for you. For example, 

on a Cisco router, you can use the command 

ip domain-name lammle.com

 to append each 

request with the 

lammle.com

 domain. If you don’t do that, you’ll have to type in the FQDN 

to get DNS to resolve the name.

f I g u r e   3 . 9    NTP

I’m connected to an atomic

clock on the Internet – here is

the exact time and date!

I have some errors to report but

I need to record them with the

correct time and date!

What is the exact time?