1. Lecture Computer Networks DNS (Domain Name System)
Prof. Dr. H.P. Großmann
Department of Information Resource Management
University of Ulm, Germany

2. DNS (Domain Name System)
is a distributed database that is used by TCP/IP applications to
translate between symbolic names and IP addresses and to
provide mail routing information.
jedi.e-technik.uni-ulm.de 
student.uni-ulm.de  orion-fddi.rz.uni-ulm.de

3. Top Level Domains
Top level domains (right-most components) were defined by a
global authority.

4. Generic Domains Domain Description com commercial organizations
edu educational institutions
gov other U.S. governmental organizations
int international organizations
mil U.S. military
net networks
org other organizations

5. Country Domains Domain Description (Examples) ch Switzerland
de Germany
fr France
in India
uk United Kingdom
us USA

6. Names
Organizations (e.g. universities, companies) apply for names in a
top-level domain:
uni-ulm.de
macdonalds.com
Organizations determine own internal structure (they can create
any internal DNS hierarchy and need not contact any central
naming authority )
e-technik.uni-ulm.de
mathematik.uni-ulm.de
Each organization maintains its own database of information and
runs name server programs that other systems across the Internet
(clients) can query.

7. Example DNS Hierarchy

8. Example
A host wants the IP address of jedi.e-technik.uni-ulm.de:

9. What does a name server X when it doesn´t know the
requested information?
 name server X contacts other name servers
Each name server has to know the top level name servers,
the lowest level server knows the answer of the request
(e.g. IP address)

10. Hierarchy of Name Servers
root name servers (13 at the moment)
the authoritative name servers for root domain
know names and IP addresses of each authoritative name server for all the top level domains
authoritative name servers for a top level domain (e.g. de - 6 at the moment)
know names and IP addresses of each authoritative name server for the second level domains
authoritative name servers for a second level domain (e.g. uni-ulm.de - 5 at the moment) ...

11. Example (2)

12. Resource Records The different types of entries in the NSs databases
(examples)
Name Description
A IP address (find IP address for a given name)
NS name server (get all NSs of a domain)
PTR pointer record (find name for a given IP address)
MX mail exchange record (find receiver of a mail address)