Sunday, 21 February 2016

DHCP: DORA Process



It is important to understand how DHCP uses the DORA process to operate. Dynamic Host Configuration Protocol (DHCP) is used to automatically assign IP address to client machines. Without DHCP, you would have to go to each machine and manually enter the address. DHCP is broadcast-based, using ports.


DHCP Dora process:
DORA in simple words is the process through which a DHCP client acquires an IP address from a DHCP server in the network

D- Discover: When a machine boots up in lan and it doesn’t have an IP address configured, it would send a DHCP discover broadcast to the network. It will have a destination IP of broadcast. It also includes it mac address encapsulated in the package. The layer netwrok layer destination would be MAC, i.e. to add devices in the network. The switch port which received the package would then forward it to all other ports in the switch except on the one from which the request is received.

O- Offer: If there is a dhcp server listening on the network, it will respond back to the DHCP request package with an offer package. The offer package is again a broadcast, but it will have the destination mac address set to the DHCP client's mac address. The source mac address will be that of the DHCP server. The offer package will contain the IP address, DNS, gateway etc as well.

R- Request: The dhcp client will get similar offers from all dhcp servers in the network and it will typically accept the first one that it receives. It will then send a request to the DHCP for the offered IP address.

A-Acknowledge: When DHCP received the DHCP Request from the client for the IP address, it will send back a DHCP acknowledge, thereby allocating that IP address to the client.

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