The DNS name space is based on a domains, which exist in a hierarchical structure much like the directory tree in a file system. A domain is the equivalent of a directory, in that it can contain either subdomains (subdirectories) or hosts (files), forming a structure called DNS tree. The DNS name space function in the same way : administrators are assigned domain names and are then responsible for specifying host names to systems within that domain. The result is that every computer on the Internet is uniquely identifiable by a DNS, name that consists of host name plus the names of all its parent domains, stretching up to the root of the DNS tree, separated by periods. Each of the names between the periods can be up to 63 characters long, with a total length of 255 characters for a complete DNS name. Domain and host names are not case sensitive, and can take any value except the null value.

1. 1
Domain Network Services (DNS)
By vikas jagtap

2. 2
Domain Network Services (DNS)
7.1 Domain Names
7.2 Authoritative Hosts
7.3 Delegating Authority
7.4 Resource Record
7.5 SOA records
7.6 DNS protocol
7.7 DHCP & Scope Resolution

3. 3
Computers are designed to work with numbers, while
humans are more comfortable working with words.
A mechanism that would permit users to refer to the
network’s computers by names, rather than by address called
as ‘Domain Name System’
IP address (32-bit) is Internet Protocol Address to identify
machines in a network. Although such addresses provide a
convenient, compact representation for specifying the source
and destination in packets sent across an internet, user prefer
to assign machines pronounceable, easily remembered names
called as ‘Domain Name.’

4. 4
The DNS name space is based on a domains, which
exist in a hierarchical structure much like the directory
tree in a file system.
A domain is the equivalent of a directory, in that it can
contain either subdomains (subdirectories) or hosts
(files), forming a structure called DNS tree.
Root
com net org
wwwwww www www www
Fig. – The Domain Name System (Tree structure)

5. 5
The DNS name space function in the same way :
administrators are assigned domain names and are then
responsible for specifying host names to systems within
that domain.
The result is that every computer on the Internet is
uniquely identifiable by a DNS, name that consists of
host name plus the names of all its parent domains,
stretching up to the root of the DNS tree, separated by
periods.
Each of the names between the periods can be up to 63
characters long, with a total length of 255 characters for
a complete DNS name.
Domain and host names are not case sensitive, and can
take any value except the null value.

6. 6
The naming scheme used in the Internet is called the Domain
Name System (DNS).
Each computer name consists of a sequence of alpha-
numeric segments separated by periods. For example, a
computer in the computer science department at Pune
University has the domain name : -
comp1.cs.pune.edu
The left-most segment if a name(comp1) is the name of an
individual computer. Other segment in a domain name
identify the group that owns the name. For example, the
segment pune gives the name of the university.

7. 7
The domain name system does not specify an exact
number of segments in each name nor does it specify
what those segments represent.
The domain name system does specify values for the
most significant segment, which is called a top-level
domain (TLD).
Following table lists the top-level domains of the DNS :
Domain Name Assigned to
com Commercial Organization
edu Educational Institution
gov US government
org Non-commercial organization
mil US military

8. 8
Top – level domains are controlled by the Internet
Corporation for Assigned Names and Numbers
(ICANN). ICANN designates one or more domain
registrars to administer a given top-level domain and
approve specific names.
Top-level domains are important because each
organization that choose to participate in the domain
name system must apply for a name under one of the
existing top level domains. Most corporations choose to
register under the com domain.
To obtain a domain, an organization must register
with an approved register. A unique domain suffix is
assigned to each organization.

9. 9
The partitioning of a namespace must be defined in a way
that support efficient name mapping and guarantee
autonomous control of name assignment.
The namespace is partitioned at the top level, and
authority for names is subdivisions is passed to designed
agents.
For example, one might choose to partition the
namespace based on site name and to delegate to each site
responsibility for maintaining names within its partition.
The topmost level of the hierarchy divides the
namespace and delegates authority for each division.

10. 10
As an example, consider a namespace with
names of the form :
local.site
Where site is the site name authorized by the central authority,
local is the part of a name controlled by the site, and the
period (“.”) is a delimiter used to separate them.
When the topmost authority approves adding a new site, X, it
adds X to the list of valid sites and delegates to site X
authority for all names that end in “.X”
Keep subdividing the namespace until each subdivision is
small enough to be manageable. Subdividing the namespace
introduces another partition of the name.
e.g.- local.group.site

11. 11
Thus, Domain name is something like a postal address,
in which the top-level domain is the equivalent of the
state, the second –level domain is the city, and the host
name is the street address.

12. 12
Domain Name begins with WWW –
Although a domain name denotes a computer, many
organizations assign domain names that reflect the service
the computer provides.
For example – If Foobar corporation provides the FTP
service, it can choose a computer to run the service, and
assign the domain name:
ftp.foobar.com
Similarly a compute that runs a Web server, can be
assigned the name:
www.foobar.com
The computer’s domain name does not need to contain
www.

13. 13
DNS servers are basically database servers that store
information about the hosts and subdomain for which they
are responsible in resource records (RPs). When you run
your own DNS server, you create a resource record for each
host name that you want to be accessible by the rest of the
network.
There are several different types of resource records used by
DNS servers, the most important of which are as follows:
SOA (Start of Authority) –
Indicates that the server is the best authoritative source for
data concerning the zone. Each zone must have an SOA
record, and only one SOA record can be in a zone.

14. 14
NS (Name Server) –
Identifies a DNS server functioning as an authority for
the zone. Each DNS server in the zone must be
represented by an NS record.
A (Address) –
Provides a name-to-address mapping that supplies an IP
address for a specific DNS name. This record type
performs the primary function of the DNS, converting
names to addresses.
PTR –
Provides an address-to-name mapping. This is the
functional opposite of an A record, used for reverse
lookups only.

15. 15
Dynamic Host Configuration Protocol (DHCP) is
designed to handle automated address assignment.
DHCP allows a computer to obtain an IP address quickly and
dynamically. To use DHCP’s dynamic address allocation
mechanism, a manager must configure a DHCP server by
supplying a set of IP addresses.]
Whenever a new computer connects to the network, the new
computer contacts the server & request an address. The server
chooses one of the address the manager specified, and
allocates that address to the computer.

16. 16
DHCP allows 3 types of address assignment –
1) Manual Configuration –
In this configuration a manager can configure a
specific address for a specific computer.
2) Automatic Configuration –
In this configuration a manager allows a DHCP server
to assign a permanent address when a computer first
attaches to the network.
3) Dynamic Configuration –
In this configuration a server “loans” an address to a
computer for a limited time.

17. 17
DHCP uses the identify of the client to decide how to
proceed. When a client contacts a DHCP server, the
client sends an identifier, usually the client’s hardware
address.
The server uses the client’s identifier and the network to
which the client has connected to determine how to
assign the client & IP address.
Thus, a manager has complete control over how address
are assigned.

18. 18
When you type a URL containing a DNS name (such as
www.microsoft.com ) into the browser’s address field and
press the ENTER key, if you look quickly at the status bar
in the lower-left corner, you’ll see a message that says
“Finding Site: www.microsoft.com.” In a few seconds,
you’ll then see a message that says “Connecting to,”
followed by an IP address. It is during this interval between
the Finding Site message and the Connecting to message
that the DNS name resolution process occurs.
From the client’s perspective, the procedure that occurs
during these few seconds consists of the application sending
a query message to its destination DNS server that contains
the name

19. 19
If you connect to the Internet, you use a DNS server each time
you enter a server name or URL into a Web browser or other
application to resolve the name of the system you specified
into an IP address.
When a standalone computer connects to an Internet service
provider (ISP), the ISP’s server usually supplies the addresses
of the DNS servers that the system will use.
TCP/IP communications are based solely on IP addresses.
Before one system can communicate with another, it must
know its IP address. Often, the user supplies a friendly name
(such as DNS name) for a desired server to a client
application. The application must then resolve that server
name into an IP address before it can transmit a message to it.
If the name resolution mechanism fails to function, no
communication with the server is possible.