Transcript 25.1 Chapter 25 Domain Name System

Chapter 25
Domain Name System
25.1
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Application Layer
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Application layer enables the user,
whether human or software to access the
network.
Provides user interfaces
Support for services: e-mail, file access
and transfer, access to system resources,
surfing WWW, network management.
Here we will discuss applications designed
as client/server pairs.
Client requests and the server responds.
IP addresses
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IP addresses are assigned to:
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Machines in a network
Resources provided on the server
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URL of any resource on the server
E-mail of any person on the server
Files etc
IP addresses are numeric, humans cannot
remember.
WHAT is DNS?
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DNS is like a phone book for the Internet.
It translates human-friendly computer
hostnames into IP addresses.
For example, the domain name
www.example.com translates to the
addresses 192.0.32.10 (IPv4) and
2620:0:2d0:200::10 (IPv6).with names
and phone numbers of our contacts.
Where the DNS database is
located on the internet?
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DNS System is a distributed database of
domain name servers present all around
the world.
Name servers do two things:
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They accept requests from programs to
convert domain names into IP addresses.
They accept requests from other name
servers to convert domain names into IP
addresses.
TASKS of DNServers
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When a request comes in, the name server can
do one of four things with it:
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It can answer the request with an IP address because it already
knows the IP address for the domain.
It can contact another name server and try to find the IP
address for the name requested. It may have to do this multiple
times.
It can say, "I don't know the IP address for the domain you
requested, but here's the IP address for a name server that
knows more than I do."
It can return an error message because the requested domain
name is invalid or does not exist.
Figure 25.1 Example of using the DNS service
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25-1 NAME SPACE
To be unambiguous, the names assigned to machines
must be carefully selected from a name space with
complete control over the binding between the names
and IP addresses.
Topics discussed in this section:
Hierarchical Name Space
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25-2 DOMAIN NAME SPACE
To have a hierarchical name space, a domain name
space was designed. In this design the names are
defined in an inverted-tree structure with the root at
the top. The tree can have only 128 levels: level 0
(root) to level 127.
Topics discussed in this section:
Label
Domain Name
Domain
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Figure 25.2 Domain name space
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Figure 25.3 Domain names and labels
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Figure 25.4 FQDN and PQDN
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Figure 25.5 Domains
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25-3 DISTRIBUTION OF NAME SPACE
The information contained in the domain name space
must be stored. However, it is very inefficient and also
unreliable to have just one computer store such a huge
amount of information. In this section, we discuss the
distribution of the domain name space.
Topics discussed in this section:
Hierarchy of Name Servers
Zone
Root Server
Primary and Secondary Servers
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Figure 25.6 Hierarchy of name servers
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Figure 25.7 Zones and domains
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Note
A primary server loads all information
from the disk file; the secondary server
loads all information from
the primary server.
When the secondary downloads
information from the primary, it is called
zone transfer.
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25-4 DNS IN THE INTERNET
DNS is a protocol that can be used in different
platforms. In the Internet, the domain name space
(tree) is divided into three different sections: generic
domains, country domains, and the inverse domain.
Topics discussed in this section:
Generic Domains
Country Domains
Inverse Domain
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Figure 25.8 DNS IN THE INTERNET
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Figure 25.9 Generic domains
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Table 25.1 Generic domain labels
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Figure 25.10 Country domains
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