HOW DHCP WORKs

HOW DHCP WORK:

DHCP is used to allocate IP address Dynamically to to the clients on a network.

DHCP Components:

DHCP Client:it is a service available to the Clients OS as well as Server OS.DHCP client service is requesting for IP address and obtaining ip from dhcp server.

DHCP Server :Define a scope (IP address Range) for the clients and lease (time duration) so that every time clients uses ip for a sepecific period and after that ip is changed .

IP ADDRESS TYPES:

static vs Dynamic ip:

Static : Fixed ip address

Dynamic : Not fixed and change frequently

DORA Process:

DHCP uses 4 Types of Packets for ip address allocation

DISCOVER Packet: 

is used to search a DHCP server on the Network, client does'nt have ip address , so they sends Discover Packets for searching a valid dhcp server on the network.if any dhcp server found , client is request ip address , if clients does'nt found any dhcp server on the network client is uses a APIPA adress automatically.

APIPA(Automatic Private IP Adress) Range between 169.254.00-169.254.255.255
if the client uses APIPA , DHCP Discover Packets continuously forward .if any dhcp server found client release the APIPA ip address and obtain ip from DHCP server.

OFFER Packet: Offer Packet is sent from Dhcp server to Client , if client found a valid dhcp server, dhcp server offer a ip address from a scope to the client .
if client have offer from more than 1 dhcp server client have a rights to decline ip address in any of the servers, 

REQUEST Packet: Request Packet is a unicast packet sends from client to the dhcp server , client accept the offer and send reqest packet to obtain ip address.

ACK Packet:ACK is a Acknowledgement packet sends from a Dhcp server to the client and with this ACK packet DHCP provide ip address ,subnet mask,Default Gateway , DNS server  ,Alternate DNS and a Lease ,by default Lease is 8 days.
dhcp uses Port No 67 ,68

IMPORTANT NOTES ABOUT DHCP

After obtaining ip address client is uses ip address , if client have 50% lease is  expired , client is again request for ip address automatically for that ip address, if
dhcp server scope is all released so dhcp server sends NACK packet to cleint .

after 85% leased is expired the client automatically sends Discover Packets again for look forward another DHCP.