DHCP allows to automatically assigning IP addresses, subnet masks, and other configuration information to client computers on the local network. When a DHCP server is available, computers that are configured to obtain an IP address automatically request and receive their IP configuration from that DHCP server upon booting. When a DHCP server is unavailable, such clients automatically adopt an alternate configuration or an Automatic Private IP Addressing (APIPA) address.

  • There are two versions of DHCP, one for IPv4 and one for IPv6. While both versions bear the same name and perform much the same purpose,
  • DHCP uses the same two ports assigned by IANA for BOOTP: UDP port 67 for sending data to the server, and UDP port 68 for data to the client. DHCP communications are connectionless in nature.
[Note: -DHCP operations fall into four basic phases: IP discovery, IP lease offer, IP request, and IP lease ack.]
 
History
DHCP was first defined as a standards track protocol in RFC 1531 in October 1993, as an extension to theBootstrap Protocol (BOOTP). The motivation for extending BOOTP was that BOOTP required manual intervention to add configuration information for each client, and did not provide a mechanism for reclaiming disused IP addresses.
                Many worked to clarify the protocol as it gained popularity, and in 1997 RFC 2131 was released, and remains as of 2011 the standard for IPv4 networks. DHCPv6 is documented in RFC 3315. RFC 3633 added aDHCPv6 mechanism for prefix delegation. DHCPv6 was further extended to provide configuration information to clients configured using stateless address auto configuration in RFC 3736.
 
DHCP Related Topic
DHCP Relay Agent
DHCP relay agent is a server that converts DHCP broadcast in to unicast. There are also RFC 1541 router which can solve multi-net reach of DHCP server.
DHCP Scope
DHCP scope is a pool of IP address and reservation created on a DHCP server.     Or
           DHCP options can be configured on reservation, scope and server level. To configure options for reservation, select it and from the actions menu choose 'Configure options'. To configure options for a scope select scope options folder and then 'Configure options'. To configure server options select server options folder and then 'Configure options'. There are more than 60 different options available for the DHCP server
[Note: - When a DHCP server uses a given scope to assign addresses to clients on the local network, the server itself needs to be assigned an address that is compatible with that scope. For example, if a scope distributes addresses within the 192.168.1.0/24 range to the local network segment, the DHCP server interface facing that segment must be assigned a static address within the same 192.168.1.0/24 range]
Super scope:-Super scope can be use to group a number of scope in to one super scope.
Server option
Server option can be used to configure client settings with specific setting this include router address, DNS address, WIN Address, Domain Name etc.
Scope Option
Scope option can be used to the provide other information like DNS, WIN address etc.
It include configuration that are apply to us a specific scope.
Vendor Class: - Vendor class includes settings for define the vendor protocols like windows 2000.
User class: -It is user define class that is use for custom class as require by the user to create custom class
 
Working Process
Normally the DHCP server provides the client with at least this basic information:
  • IP Address
  • Subnet Mask
  • Default Gateway
  • Other information can be provided as well, such as Domain Name Service (DNS) server addresses and Windows Internet Name Service (WINS) server addresses. The system administrator configures the DHCP server with the options that are parsed out to the client.
When a client is initialized for the firs t time after it is configured to receive DHCP information, it initiates a conversation with the server.
 
The detailed conversation between DHCP client and DHCP server is as follows:
DHCP-Discover
The client sends a DHCPDISCOVER packet. The following is an excerpt from a network monitor capture showing the IP and DHCP portions of a DHCPDISCOVER packet. In the IP section, you can see the Destination address is 255.255.255.255 and the Source address is 0.0.0.0. The DHCP section identifies the packet as a Discover packet and identifies the client in two places using the physical address of the network card. Note the values in the CHADDR field and the DHCP: Client Identifier field are identical.
DHCP-Offer
The DHCP server responds by sending a DHCPOFFER packet. In the IP section of the capture excerpt below, the Source address is now the DHCP server IP address, and the Destination address is the broadcast address 255.255.255.255. The DHCP section identifies the packet as an Offer. The YIADDR field is populated with the IP address the server is offering the client. Note the CHADDR field still contains the physical address of the requesting client. Also, we see in the DHCP Option Field section the various options being sent by the server along with the IP address. In this case the server is sending the Subnet Mask, Default Gateway (Router), Lease Time, WINS server address (NetBIOS Name Service), and the NetBIOS Node Type.
DHCP-Request
The client responds to the DHCPOFFER by sending a DHCPREQUEST. In the IP section of the capture below, the Source address of the client is still 0.0.0.0 and the Destination for the packet is still 255.255.255.255. The client retains 0.0.0.0 because the client hasn't received verification from the server that it's okay to start using the address offered. The Destination is still broadcast, because more than one DHCP server may have responded and may be holding a reservation for an Offer made to the client. This lets those other DHCP servers know they can release their offered addresses and return them to their available pools. The DHCP section identifies the packet as a Request and verifies the offered address using the DHCP: Requested Address field. The DHCP: Server Identifier field shows the IP address of the DHCP server offering the lease.
DHCP-Acknowledgment
The DHCP server responds to the DHCP-REQUEST with a DHCP-ACK, thus completing the initialization cycle. The Source address is the DHCP server IP address, and the Destination address is still 255.255.255.255. The YIADDR field contains the client's address, and the CHADDR and DHCP: Client Identifier fields are the physical address of the network card in the requesting client. The DHCP Option section identifies the packet as an ACK.

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