DHCP (Dynamic Host Configuration Protocol)

DHCP (Dynamic Host Configuration Protocol) is a protocol that lets network administrators run centrally and automate the assignment of Internet Protocol (IP) addresses in an organization's network. but a machine uses Internet's set of protocol (TCP/IP), all machine that can connect to the Internet requires a unique IP address. while an organization sets up its computer users with a connection to the Internet, an IP address have to be assigned to every machine. Without DHCP, the IP address have to be entered manually at every computer and if computers move to a different location in an additional part of the network than a new IP address must be entered. DHCP lets a network admin. supervise and distribute IP addresses from a middle point and automatically sends a new IP address when a computer is plugged into a different place in the network.

IP Address Allocation Mechanism

DHCP supports 3 mechanisms for IP address allotment.

• Automatic allocation: DHCP assigns a everlasting IP address to a host.
• Dynamic allocation: DHCP assigns an IP address to a host for a limited period of time (till the host explicitly relinquishes the address).

• Manual allocation: Host's IP address is assigned by the network administrator, and DHCP is used simply to transmit the assigned address to the host. A particular network will use 1 or more than  one of these methods, depending on the policies of the network administrator.

Messages Used by DHCP

• DHCP Discover - Client broadcast to find available servers. It is assumed at smallest amount one of the servers will have resources to fulfill the request.(might include additional pointers to specific services required for example particular subnet, minimum time limit a lot more).
• DHCP Offer - Here the machine called Server to client in response to DHCP find out with offer of configration parameters.

• DHCP Request - Client broadcast to servers requesting offered parameters from one server and implicitly declining offers from all others.( also significant in case of lease renewal if the alloted time is about to expire ).

• DHCP Decline - Client to server representing configration parameters invalid.
• DHCP Release - In this case Client to server relinquishing network address and cancelling current lease.( in case of a graceful shut down DHCP server is sent a DHCP Release by the host machine).

• DHCP Ack - Server to client with configration parameters, with dedicated Network address.

• DHCP Nack - Server to client refusing request for configratin parameters (eg. requested network address already allocated).

Timers Used

Note that lease time is the time provided by the server for which the services have been given to the client.

• Lease Renewal Timer - When this timer expires system will ask the server for more time sending a DHCP Request.
• Lease Rebinding Timer - At whatever time this timer expires, we have not been getting any response from the server and so we can think that the server is down. as a result send a DHCP Request to all the servers using IP Broadcast facility. This is only point of difference among Lease renewal and rebinding.

• Lease Expiry Timer - At whatever time this timer expires, the machine will have to start crashing as the host does not have a valid IP address in the network.

Timer Configuration Policy

The timers have this common setting which can be configured depending upon the usage pattern of the network. for example setting has been shown below.

Lease Renewal = 50 percent. Lease time Lease Rebinding = 87.5 percent. Lease time Lease Expiry = 100 percent.Lease time

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