The document discusses the Domain Name System (DNS) which maps host names to IP addresses. It explains that DNS uses a hierarchical and distributed database to store mappings in a scalable way. The DNS database is divided into zones managed by name servers. Queries are resolved by traversing the hierarchy from the root servers down to the authoritative name server for the requested domain.
2. DOMAIN NAME SYSTEM (DNS)
People prefer to use names instead of the addresses so the Internet
needs to have a directory system that can map a name to an address.
As internet is so huge today so a central directory can not hold that
much mapping and disadvantage is if central computer fails whole
communication will collapse.
So information is distributed among many computers in this world.
In this method the host that needs mapping can contact to its
closest computer holding the needed information. This method is
used by DOMAIN NAME SYSTEM (DNS).
3. CONTINUED….
The following six steps map the host to an IP address.
1. The user passes the host name to the file transfer client.
2. The file transfer client passes the host name to the DNS client.
3. Each computer after booting Knows the address of one DNS server.
The DNS client sends a message to a DNS server with a query that
gives the file transfer server user name using the known IP address
of the DNS server.
4. The DNS server responds with the IP address of the desired file
transfer server.
5. The DNS server passes the IP address of the file transfer client.
6. The file transfer client now uses the received IP address to access
the file transfer server.
4.
5. NAME SPACE
A ‘Name Space’ that maps each address to a unique name can be
organized in two ways:
1. Flat Name Space: A name in this space is a sequence of characters
without structure. The names may or may not have a common
section if they do, it has no meaning. Disadvantage can not be
used in large systems because it must be centrally controlled to
avoid ambiguity and duplication.
2. Hierarchical Name Space: In this space each name is made of
several parts. The first can define the name of the organization,
second can define its nature, 3rd can define the departments and
so on. In this case the authority to assign and control can be
decentralized.
the first organization can choose first.com and now the hosts can
choose name of
6.
7. DOMAIN NAMES AND LABELS
LABEL: Each node in a tree has a label which is a string of maximum
63 characters. The root label is null string (Empty). DNS requires
children of a node have different label which guarantees the
uniqueness of the domain name.
DOMAIN NAME: Each node in a tree has a domain name. A full
domain name is a sequence of labels separated by dots (.) Always
read from node up to the root.
Fully Qualified Domain Name (FQDN) is that if a name ends with a
dot that means null label.
Partially Qualified Domain Name(PQDN) is a name if it does not end
with a null string i.e. it starts from node but does not end at root.
(Can be made FQDN by supplying the missing part called suffix).
8.
9. DOMAIN
A domain is a subtree of the domain name space. The name of the
domain is the name of the node at the top of the subtree. (Reliability)
10. HIERARCHY OF NAME SERVERS
The whole information can not be stored in the single computer.
Solution is divide the whole space into many domains based on the
first level.
DNS allows domains to be further divided into the sub-domains
where each server is responsible of for its domain that can be large or
small.
The domain server that has sub-domain servers will must have
some way to contact to its sub-domains.
11.
12. ZONE
What a server is responsible for or has authority over is called ZONE.
It is contiguous part of entire tree.
If a server accepts responsibility of a domain and does not divide it
into sub domains then domain and zone are same thing. The server
will keep record of nodes in a database called a zone file.
If a server divides into subdomains then server and domains are two
different things. Information about the noes in the subdomains is
stored in the servers at lower levels, with the original server keeping
some sort of reference to the lower levels.
13.
14. ROOT SERVERS
A server whose Zone consists of the whole tree. Keeps reference to
other servers in its zone and delegates it authority to other servers.
1. Primary Server: A server that stores a file about the zone for which
it is in authority. It is responsible for creating, maintaining and
updating the zone file. And stores that file on local disk.
2. Secondary Server: It only copies the information in the primary
server and stores it on its own disk. It is not responsible for
updating etc. If that is required it must be done through primary
server. A sort of backup if primary server fails.
A server can be primary server for a specific zone and secondary
server for another zone therefore we must be careful about which
zone we refer to.
15. DNS IN THE INTERNET
Generic Domain:
Country Domain
Inverse Domain
Inverse domain are difficult to track, which could be used to find the
name of a host when given IP address.
