How DHCP Works?

Dynamic Host Configuration Protocol (DHCP) is a protocol (set of instructions) used by computers to dynamically attain an IP-Address & it’s required networking configuration and then maintaining them automatically. 

Overview:

In this protocol client computers will contact a Server computer that is designated as DHCP server.  The DHCP server will maintain a Database of free, assigned and reserved IP-addresses and other networking services configuration.  Whenever there comes a DHCP request from clients on the network the DHCP server will respond with available networking configuration.  If there are no free IP-addresses at DHCP server, the client request would be sent over to other DHCP servers on the same network. 

Below is a pictorial representation of clients contacting DHCP server over the network.

IP-Address assignment process:

Below are the steps that involve in a process (DORA) of clients attaining an IP-address from the DHCP server:

1. Discover:  Client computers broadcast a network request/packet looking for DHCP servers
2. Offer:  DHCP servers in that network will offer/respond with free IP-address 
3. Request:  Client responds to the first DHCP server asking for the lease (reserve for that computer) of that IP-address
4. Acknowledgement:  DHCP server issues the lease and acknowledges to the client with the lease availability

IP-Address Renewal process:

DHCP lease Renewal involves only last two steps of the DORA process that is RA.  This is because the client already knows the DHCP server to be contacted and DHCP server as well knows client with it’s existing IP-address.

Renewal Process:

1. Client attempts to renew at half of the lease period (default lease period is #8 days). 
2. If DHCP server is unavailable during the renew, client will again attempt to renew at every half of the remaining lease period. 
3. Client attempts to renew with the same DHCP server till the expired lease period  reaches 87 ½%
4. At which point, client attempts to renew with any available DHCP server
5. if no DHCP servers are available for renewal, clients altogether starts IP-address assignment (DORA) process afresh

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