Using Names and the Domain Name System
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Can you imagine a world in which every time you used an application, you had to think
about the other computer and refer to it by IP address? Instead of using easy names like
google.com or facebook.com, you would have to remember and type IP addresses, like
184.108.40.206. Certainly, that would not be user friendly and could drive some people away
from using computers at all.
Thankfully, TCP/IP defines a way to use hostnames to identify other computers. The user
either never thinks about the other computer or refers to the other computer by name.
Then, protocols dynamically discover all the necessary information to allow communications
based on that name.
For example, when you open a web browser and type in the hostname www.google.com,
your computer does not send an IP packet with destination IP address www.google.com; it
sends an IP packet to an IP address used by the web server for Google. TCP/IP needs a way
to let a computer find the IP address used by the listed hostname, and that method uses the
Domain Name System (DNS).
Enterprises use the DNS process to resolve names into the matching IP address, as shown
in the example in Figure 4-13. In this case, PC11, on the left, needs to connect to a server
named Server1. At some point, the user either types in the name Server1 or some application
on PC11 refers to that server by name. At Step 1, PC11 sends a DNS message—a DNS
query—to the DNS server. At Step 2, the DNS server sends back a DNS reply that lists
Server1’s IP address. At Step 3, PC11 can now send an IP packet to destination address
10.1.2.3, the address used by Server1.
Figure 4-13 Basic DNS Name Resolution Request
Note that the example in Figure 4-13 shows a cloud for the TCP/IP network because the
details of the network, including routers, do not matter to the name resolution process.
Routers treat the DNS messages just like any other IP packet, routing them based on the
destination IP address. For example, at Step 1 in the figure, the DNS query will list the DNS
server’s IP address as the destination address, which any routers will use to forward the packet.
Finally, DNS defines much more than just a few messages. DNS defines protocols, as well
as standards for the text names used throughout the world, and a worldwide set of distributed
DNS servers. The domain names that people use every day when web browsing,
which look like www.example.com, follow the DNS naming standards. Also, no single
DNS server knows all the names and matching IP addresses, but the information is distributed
across many DNS servers. So, the DNS servers of the world work together, forwarding
queries to each other, until the server that knows the answer supplies the desired
IP address information.