What is the DHCP message which informs the server the client will not be using the assigned IP address anymore?

DHCP uses the client/server model. A DHCP client sends a message to a DHCP server to request configuration parameters, such as the IP address, subnet mask, and default gateway address. The DHCP server responds with a message carrying the requested configuration parameters. DHCP messages sent between clients and servers share an identical fixed format header and a variable format area for options.

• DHCP Message Format
• DHCP Options

DHCP Message Format

Figure 4-2 shows the DHCP message format.

Figure 4-2 DHCP message format

Table 4-1 describes the fields in a DHCP message.

Table 4-1 DHCP message fields

DHCP Options

In the DHCP options field, the first four bytes are decimal numbers 99, 130, 83 and 99, respectively. This is the same as the magic cookie defined in standard protocols. The remaining bytes identify several options as defined in standard protocols. One particular option, the DHCP Message Type option (Option 53), must be included in every DHCP message. Option 53 defines DHCP message types, including the DHCPDISCOVER, DHCPOFFER, DHCPREQUEST, DHCPACK, DHCPNAK, DHCPDECLINE, DHCPRELEASE, and DHCPINFORM messages.

• DHCP message types

  Table 4-2 lists the DHCP message types.

  Table 4-2 DHCP message types

  Type	Description
  DHCP DISCOVER	A DHCP Discover message is broadcast by a DHCP client to locate a DHCP server when the client attempts to access a network for the first time.
  DHCP OFFER	A DHCP Offer message is sent by a DHCP server in response to a DHCP Discover message. A DHCP Offer message carries various configuration parameters.
  DHCP REQUEST	A DHCP Request message is sent in the following conditions: After a DHCP client is initialized, it broadcasts a DHCP Request message in response to the DHCP Offer message sent by a DHCP server. After a DHCP client restarts, it broadcasts a DHCP Request message to confirm the configuration including the assigned IP address. After a DHCP client obtains an IP address, it unicasts or broadcasts a DHCP Request message to update the IP address lease.
  DHCP ACK	A DHCP ACK message is sent by a DHCP server to acknowledge the DHCP Request message from a DHCP client. After receiving a DHCP ACK message, the DHCP client obtains the configuration parameters including the IP address.
  DHCP NAK	A DHCP NAK message is sent by a DHCP server to reject the DHCP Request message from a DHCP client. For example, if a DHCP server cannot find matching lease records after receiving a DHCP Request message, it sends a DHCP NAK message indicating that no IP address is available for the DHCP client.
  DHCP DECLINE	A DHCP Decline message is sent by a DHCP client to notify the DHCP server that the assigned IP address conflicts with another IP address. Then the DHCP client applies to the DHCP server for another IP address.
  DHCP RELEASE	A DHCP Release message is sent by a DHCP client to release its IP address. After receiving a DHCP Release message, the DHCP server can assign this IP address to another DHCP client.
  DHCP INFORM	A DHCP Inform message is sent by a DHCP client to obtain other network configuration parameters such as the gateway address and DNS server address after the DHCP client has obtained an IP address.

• DHCP options

  The options field in a DHCP message carries control information and parameters that are not defined in common protocols. When a DHCP client requests an IP address from a DHCP server that has been configured to encapsulate the options field, the server returns a DHCP Reply packet containing the options field. Figure 4-3 shows the options field format.

  Figure 4-3 Options field format

  The options field consists of the sub-fields Type, Length, and Value. Table 4-3 describes these sub-fields.

  Table 4-3 Sub-fields in the DHCP options field

  Sub-field	Length	Description
  Type	1 byte	Type of the message content
  Length	1 byte	Length of the message content
  Value	Determined by the Length field value	Message content

  The type value of the options field ranges from 1 to 255. Table 4-4 lists common DHCP options.

  Table 4-4 Options in DHCP messages

  Options ID	Description
  1	Subnet mask
  3	Gateway address
  6	DNS address
  15	Domain name
  33	Group of classful static routes After a DHCP client receives DHCP messages with this option, it adds the classful static routes contained in the option to its routing table. In classful routes, masks of destination addresses are natural masks and cannot be used to divide subnets. If Option 121 exists, Option 33 is ignored.
  44	NetBIOS name
  46	NetBIOS object type
  50	Requested IP address
  51	IP address lease
  52	Additional option
  53	DHCP message type
  54	Server identifier
  55	Parameter request list The DHCP client uses this option to request specified configuration parameters
  58	Lease renewal time (Time1), which is 50% of the lease time
  59	Lease renewal time (Time2), which is 87.5% of the lease time
  121	Group of classless routes After a DHCP client receives DHCP messages with this option, it adds the classless static routes contained in the option to its routing table. Classless routes can have destination address masks composed of any values and these masks can be used to divide subnets.

  The use of the options field differs depending on its function.

  For more information about common DHCP options, see standard protocols.

• Customized DHCP options

  Some options are not defined in standard protocols. Option 43 and Option 82, which are customized options, are described as follows:

  • Option 43

    Option 43 is called the vendor-specific information option. Figure 4-4 shows the Option 43 format.

    Figure 4-4 Option 43 format

    DHCP servers and DHCP clients use Option 43 to exchange vendor-specific information. When a DHCP server receives a DHCP Request message with parameter 43 encapsulated in Option 55, the server encapsulates Option 43 in a DHCP Reply message and sends it to the DHCP client.

    To implement extensibility and allocate more configuration parameters to DHCP clients, Option 43 supports sub-options, which are shown in Figure 4-4. Sub-options follow a similar format to that used for Options. They contain a Type, Length, and Value sub-field. In the Type sub-field, the value 0x01 indicates the Auto-configuration server (ACS) parameter, the value 0x02 indicates the SP ID, and the value 0x80 indicates the Preboot execution environment (PXE) server address.

    If a device functions as a DHCP client, it can obtain the following information using Option 43:

    • ACS parameters, including the uniform resource locator (URL), user name, and password

    • SP ID that the Customer Premises Equipment (CPE) notifies the ACS of so that the ACS can select configuration parameters from the specified SP

    • PXE server address, which is used by a DHCP client to obtain the Bootfile or control information from the PXE server

  • Option 82

    The Option 82 field is called the DHCP relay agent information field. It records the location of a DHCP client. A DHCP relay agent or a DHCP snooping-enabled device appends the Option 82 field to a DHCP Request message sent from a DHCP client and forwards the message to a DHCP server.

    Servers use the Option 82 field to learn the location of DHCP clients, implement client security and accounting, and make parameter assignment policies, allowing for more flexible address allocation.

    The Option 82 field contains a maximum of 255 sub-options. If the Option 82 field is defined, at least one sub-option must be defined. Currently, the following sub-options are supported: sub-option 1 (Circuit ID), sub-option 2 (Remote ID) and sub-option 6 (Subscriber ID).

    The content of the Option 82 field is not uniformly defined, and vendors fill in the Option 82 field as needed.

    The device supports the following Option 82 field formats:

    • Type1: This is the Telecom format of Option 82.

    • Type2: This is the NMS format of Option 82.

    • Cn-telecom: This is the Option 82 format defined by China Telecom.

    • Self-define: This is the user-defined format of DHCP Option 82.

What is a DHCP inform message?

Which DHCP message is used by a client to inform a DHCP server that it accepts the assigned IP address?

What type of message does a DHCP server send to a client to verify if an IP address has been assigned to a client?

What are the 4 types of messages sent and received in DHCP?