The document discusses the Domain Name System (DNS) which assigns meaningful names like springer.com to IP addresses and maps domain names to machines. DNS provides services like host and mail server aliasing and load distribution. It describes DNS components like domains, name servers, caches and zones. DNS uses a hierarchical name space structure and domain names are composed of labels separated by dots. Name resolution can be done recursively or iteratively by mapping names to addresses and vice versa. DNS messages have a specific format with fields for identification, flags, and number of question, answer, authority and additional records.

1. Domain Name System
• The goal is to assign meaningful high level
names to a large set of machines and handle
the mapping of those names to a machines IP
address.
• Domain names are case insensitive.
• com and COM mean the same thing.
• Service provided by DNS
• Host aliasing
• Mail server aliasing
• Load distribution.

2. Components of DNS
• Domain(.com)
• Domain name(springer.com)
• Name server
• Name resolver
• Name cache
• Zone

3. DNS in the Internet

4. Generic Domains

5. Generic Domain Labels

6. Country Domains

7. Inverse Domain

8. Namespaces
• Flat name spaces
• Hierarchical name spaces
Flat Name Space
 A name is assigned to an address.
 Advantage: Names were convenient & short.
 Disadvantage: Cannot generalize to large set of machines
Hierarchical Name Space
 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.

9. Domain Name Space

10. Domain Names & Labels

11. Hierarchy of Name Server

12. Domain Name & Zone

13. Resolution
• Mapping a name to an address or an address
to a name is called name-address resolution.
• Two types
– Recursive resolution
– Iterative resolution

14. Recursive Resolution

15. TCP/IP Protocol Suite 15
Iterative resolution
1
2
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4
5
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9
10

16. Message/Header Format
Messages are sent between domain clients & domain
servers with a specific format.
Query & Response Messages

17. General Format of DNS
Identification – 16 bit field & unique value used by the client to match responses to
queries
Flags – Collection of subfields that define the type of messages & type of answers
requested & so on.
No. of Question Record – Contains no. of queries in the question section of the
message.
No. of Answer record – Contains no. of answer records in the answer section of the
response message.
No. of authority records- Contains no. of authority records in the authorative section
of the response message.
No. of Additional records – Contains no. of additional records in the additional
section of the response message.

18. Flag field
QR ( 0 – Message is a query, 1- It is response
Opcode – (0-standard query, 1- Inverse query, 2- Server status request)
AA – Authoritative answer
TC – Truncated
RD – Recursive query
RA – Recursive available
Three zeros.
R Code – Return code