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Dated: Jul. 06, 2012

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Overview of DHCP Servers

Dynamic Host Configuration Protocol or DHCP servers are the dedicated computers that have network operating systems installed on them and are responsible for providing automatic IP addresses to all the client computers that are configured to obtain the addresses automatically.

DHCP is the role that administrators install on the network operating systems before the servers start assigning dynamic IP addresses to the client computers.

Since DHCP servers remarkably reduce administrative overheads by eliminating the requirement of assigning static IP addresses to the client computers manually, they are configured in almost every medium or large scale organization.

How DHCP Servers Work

As soon as DHCP server role is installed on a network operating system, if the network setup is domain-client based, the server verifies if it is authorized with the domain controller. If it is authorized, it starts assigning dynamic IP addresses to all the domain clients on the network. If the network setup is workgroup type, the DHCP server starts assigning addresses to the DHCP clients.

DHCP server assigns IP addresses to the client computers in four steps. The steps are D O R A  and their descriptions are given as below:

• D – D stands for DHCP Discover packet. DHCP Discover is a broadcast packet sent from DHCP clients when it starts, to search the available DHCP servers in the network. Since the packet is broadcasted, it is received by every computer in the network but only DHCP servers respond to it.
   
• O – O stands for DHCP Offer packet. DHCP Offer is a unicast packet that is sent to the client computer that has broadcasted the DHCP Discover packet. DHCP server sends this packet offering IP address that is available in its DHCP address pool. All DHCP servers that are available on the network respond in the same manner to the broadcasted DHCP Discover packet.
   
• R – R stands for DHCP Request packet that is sent from the DHCP client to the DHCP server requesting it to assign the offered IP address. This is a unicast packet that is sent to the DHCP server whose DHCP Offer packet is first received by the client computer.
   
• A – A stands for DHCP Acknowledge packet that is sent from the DHCP server to the DHCP client computer confirming the assignment of the offered and requested IP address.

DHCP servers assign dynamic IP addresses to the client computers for a specific duration of time. This time is technically known as lease duration. Once the lease durations of the assigned IP addresses are expired, the IP addresses must be renewed from the DHCP servers in order to stay connected to the networks. However as the default configuration, every client computer sends IP address renewal request to the DHCP server after 50% of the lease duration is lapsed.

Unavailability of DHCP Servers and APIPA

In case a network setup does not have DHCP server, administrators are required to assign static IP addresses to the client computers in order to connect them to the network. However there might be cases when the DHCP server is available on the network and the client computers are configured to obtain the IP addresses from the DHCP server but because of any reason the DHCP server is not available. Possible reasons might be DHCP server failure, disconnected cable, etc. In such cases, the client computers automatically assign IP addresses to themselves. The IP addresses that client computers assign to themselves belong to Class B address range with 169.254.0.0 network ID and 255.255.0.0 as their default subnet masks. This automatic assignment of IP addresses to themselves as technically called Automatic Private IP Addressing or APIPA.

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