DHCP Client jj] DHCP client broadcasts a DHCPDISCOVER packet
2\ DHCP servers broadcast a DHCP OFFER packet
J] DHCP client broadcasts a DHCPREQUEST packet
%- 'i/' 4 I DHCP Serverl broadcasts a DHC PACK packet
ILLEGAL FOR NON-TRAINER USE
DHCP uses a four-step process to lease IP addressing information to DCHP clients. The four steps are named after the DHCP packet types.
1. DHCP discover
2. DHCP offer
3. DHCP request
4. DHCP acknowledgement or DHCP negative acknowledgement
The DHCP client broadcasts a DHCPDISCOVER packet
The DHCP lease generation process is the process by which the DHCP client receives IP addressing configuration data from the DHCP server.
The DHCP client broadcasts a DHCPDISCOVER packet to locate a DHCP server. A DHCPDISCOVER packet is a message that DHCP clients send the first time that they attempt to log onto the network and request IP address information from a DHCP server.
There are two ways that the lease generation process can begin. The first occurs when a client computer either starts up or initializes TCP/IP for the first time. The second occurs when a client attempts to renew its lease and is denied. (For example, a client can be denied a renewal when you move it to another subnet.)
The DHCP server broadcasts a DHCPOFFER packet
The DHCP client broadcasts a DHCPREQUEST packet
The DHCP server broadcasts a DHCPOFFER packet to the client. A DHCPOFFER packet is a message that DHCP servers use to offer the lease of an IP address to a DHCP client when it starts on the network.
Each responding DHCP server reserves the offered IP address in order not offer it to another DHCP client before the requesting client's acceptance.
If the client does not receive an offer after four requests, it uses an IP address in the reserved range from 169.254.0.1 to 169.254.255.254. The use of one of these auto-configured IP addresses ensures that clients located on a subnet with an unavailable DHCP server are able to communicate with each other. The DHCP client continues to attempt to find an available DHCP server every five minutes. When a DHCP server becomes available, clients receive valid IP addresses, allowing those clients to communicate with hosts both on and off their subnet.
The DHCP client broadcasts a DHCPREQUEST packet. A DHCPREQUEST packet is a message that a client sends to the DHCP server to request or renew the lease of the client's IP address.
The DHCP client responds to the first DHCPOFFER packet that it receives by broadcasting a DHCPREQUEST packet to accept the offer. The DHCPREQUEST packet includes the identification of the server whose offer the client accepted. All other DHCP servers then retract their offers and retain their IP addresses for other IP lease requests.
The DHCP server broadcasts a DHCPACK packet to the client. A DHCPACK packet is a message that the DHCP server sends to a client to acknowledge and complete a client's request for leased configuration. This message contains a valid lease for the IP address and other IP configuration data.
When the DHCP client receives the acknowledgment, TCP/IP initializes by using the IP configuration data that the DHCP server provides. The client also binds the TCP/IP protocol to the network services and network adapter, permitting the client to communicate on the network.
The DHCP server sends a DHCP negative acknowledgement (DHCPNAK packet) if the IP address that was offered is no longer valid or is now in use by another computer. The client must then begin the lease process again.
Important A DHCP server and a DHCP client communicate by using User Datagram Protocol (UDP) ports 67 and 68. Some switches do not properly forward DHCP broadcasts by default. For DHCP to function correctly, you may need to configure these switches to forward broadcasts over these ports.
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