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Communication Protocols - DHCP and DNS

Published on Tuesday, December 22, 2015
communication protocols
We'll be starting with communication protocols today as a part of computer networks. Any doubts/suggestions- comments section is all yours!

==>> This article is a part of PK Series (IT)

As we all now all computers on internet are accessed via IP addresses, it becomes really hard to remember IP addresses and assign them statically in large networks. As a solution to these problems, we came up with concepts of DHCP and DNS.

Domain Name System (DNS)

DNS is usually used to translate a host name into an IP address. E.g. -  when you hit '' in your browser, it actually hits IP on internet and fetches us the home page (may vary depending upon geographies as google has many servers). Domain names comprise a hierarchy so that names are unique and easy to remember. Some points about DNS are as below:
  • Stores other information other than simple host-IP mappings. 
  • Root servers are maintained throughout the world and are responsible for all of top level domains. When you register a domain, an entry for that domain is added to appropriate root server. 
  • An administration zone is a subtree of DNS tree that is independently managed. E.g- ("") and ("") belong to one independent zone for administration. 
  • A zone must provide for multiple name servers which record the members in the domain. 
  • The kind of records that can be requested are- Address Translation, Caching Information, Mail Server Information.
  • DNS uses distributed database as there is one primary server for a domain and typically a number of secondary servers containing replicated databases. 
  • The organization that owns the domain name is responsible for running a DNS server that provides for the mapping. 
  • A DNS client is called a resolver. 
  • Most Unix workstations have the file /etc/resolv.conf that contains local domain and addresses of DNS servers for that domain. 
  • nslookup, usually available on unix workstations, is an interactive resolver that allows the user to communicate directly with a DNS server. (hit nslookup followed by for a feel of it)
  • If a server is asked to provide the mapping for a host outside its domain, it finds the nameserver for target domain and asks it to provide the translation. 
  • Common DNS library functions - 'gethostbyname' & 'gethostbyaddr'
  • Uses port number 53
A DNS tree looks like:

Dynamic Host Configuration Protocol (DHCP) 

DHCP is used to dynamically assign configuration parameters (IP address, subnet mask, default gateway, DNS information) to network devices. Some points about DHCP are as below:
  • A host without a valid IP address communicated with DHCP server, DHCP server then dynamically allocates address. It may reuse the addresses. 
  • Eliminates the need for manual host configuration.
  • It can co-exist with other pre assigned IP addresses and hots which don't participate in DHCP. 
  • DHCP client uses port number 68.
  • Messages exchanged between Client and Server are as follows:
      • Client sends a DISCOVER message to all computers. 
      •  DHCP server upon receiving that message sends an OFFER
      • Client then sends REQUEST to obtain an IP address.
      • Server responds with ACK (request accepted) and NACK (something wrong with REQUEST)
      • DECLINE message may be sent by client in case it finds that IP address assigned to it is already in use.
      • INFORM message may be send by client in case IP is statically configured on client and it needs other configuration settings only. 
      • Finally, Client may send RELEASE when its done with using that IP, not necessary. 
Dynamic Host Configuration Protocol

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