1. BAI513 - PROTOCOLS
DHCP
BAIST – Network Management

2. Objectives
At the end of this presentation, the student will be able to:
Describe the history of DHCP
Describe the DHCP address discovery process
Describe the fields contained in the DHCP header and their functions.

3. History of DHCP
DHCP is based on BOOTP and maintains some backward compatibility.
The main difference is that BOOTP was designed for manual pre-configuration of the host information in a server database, while DHCP allows for dynamic allocation of network addresses and configurations to newly attached hosts.
Additionally, DHCP allows for recovery and reallocation of network addresses through a leasing mechanism.

4. DHCP Defined
The Dynamic Host Configuration Protocol (DHCP) is an Internet protocol for automating the configuration of computers that use TCP/IP.
DHCP can be used to automatically assign IP addresses, to deliver TCP/IP stack configuration parameters such as the subnet mask and default router, and to provide other configuration information such as the addresses for printer, time and news servers.

5. DHCP Components DHCP consists of two components:
A protocol for delivering host-specific configuration parameters from a DHCP server to a host
A mechanism for allocation of network addresses to hosts.

6. Allocating IP Addresses
DHCP supports 3 mechanisms for allocating IP addresses.
Automatic - assigns a permanent IP address to a client
Dynamic - assigns an IP address to a client for a limited period of time
Manual - a client's IP address is assigned by the network administrator, and DHCP is used simply to convey the assigned address to the client

7. DHCP Address Discovery
The DHCP Address Discovery process actually uses seven packets:
DHCPDISCOVER packet
DHCPOFFER packet
DHCPREQUEST packet
DHCPACK packet
Client (Win2k) then sends 3 ARP packets to validate the IP address assigned by the DHCP Server

8. The Discover Packet Client to Server
When a DHCP client has no IP address (booting for the first time, or after a lease expires), it must broadcast a DHCP Discover Packet onto the network.
The DHCP Discover Packet Is Always Sent with a destination Hardware (FF:FF:FF:FF) and IP Broadcast ( ) addresses.

9. The DHCPDISCOVER Packet
If the DHCP client was on the network before, the client also defines a preferred address—typically the client prefers the last address it used
In the DHCP Discover packet, the Message Type value is one—this indicates that this packet is a DHCP Discover packet
The Client Identifier field value is based on the client’s hardware address
NOTE: Parameters found in DHCP Options

10. The DHCPOFFER Packet Server to Client
The DHCP server sends the DHCPOFFER packet (unicast destination address) to offer an IP address to the DHCP client
Each responding DHCP server includes an unique server identifier in the option field
The DHCPOFFER packet includes the IP address that is offered to the client, and sometimes answers to the requested options in the DHCPDISCOVER packet

11. The DHCPREQUEST Packet
Client to Server
Once the DHCPOFFER packet is received, the client can either accept the offer by issuing a DHCPREQUEST (broadcast) packet,
If a host receives a multiple offers the server ID in the DHCPREQUEST identifies which server, and offer the client is using.

12. The DHCPACK Packet Server to Client
The DHCPACK packet (unicast) includes some answers to the client’s request for information, such as:
The client subnet mask is
The client’s default gateway address is
The client’s DNS server address is

13. Additional DHCP Packets
Additional DHCP packets that may be used include:
DHCPNAK Server to client indicating client's notion of network address is incorrect (e.g., client has moved to new subnet) or client's lease as expired 
DHCPDECLINE - Client to server indicating network address is already in use. 
DHCPRELEASE - Client to server relinquishing network address and cancelling remaining lease. 
DHCPINFORM - Client to server, asking only for local configuration parameters; client already has externally configured network address.

14. DHCP Frame The DHCP Frame is comprised of:
The DHCP Header and DHCP Data Fields are encapsulated inside a UDP Header.
The UDP Header is encapsulated inside a standard IPv4 Header which is encapsulated inside a standard MAC Header with a CRC Trailer.

15. DHCP Header Structure

16. DHCP Header Fields Opcode Hardware Type
This 1-byte field indicates whether this packet is a DHCP request (0x01) or DHCP reply (0x02)
Hardware Type
This 1-byte field identifies the hardware address type, and matches the values assigned for ARP hardware type definitions

17. DHCP Header Fields Hardware Address Length Hop Count
This 1-byte field indicates the length of the hardware address
Hop Count
This field is set to zero by the client, and may be used by relay agents as they assist a client in obtaining an IP address and/or configuration information

18. DHCP Header Fields Transaction ID Number of Seconds
This 4-byte field contains a random number selected by the client, and is used to match requests and responses between the client and server
Number of Seconds
This 2-byte field indicates the number of seconds that elapsed since the client began requesting a new address, or renewal of an existing address

19. DHCP Header Fields Flags Client IP Address
The first bits of the 2-byte Flags field can be toggled to one to indicate that the DHCP client cannot accept unicast MAC layer datagrams before the IP software is completely configured
Client IP Address
The DHCP client fills in this 4-byte field with its IP address after it is assigned and bound to the IP stack

20. DHCP Header Fields Your IP Address Server IP Address
This 4-byte field contains the address being offered by the DHCP server
Server IP Address
This 4-byte field contains the IP address of the DHCP server to use in the boot process

21. DHCP Header Fields Gateway IP Address Client Hardware Address
This 4-byte field contains the address of the DHCP relay agent, if one is used
Client Hardware Address
This 16-byte field contains the hardware address of the client

22. DHCP Header Fields Server Host Name Boot Filename
This 64-byte field can contain the server host name, but such information is optional
This field can contain a null-terminated string (all zeroes)
Boot Filename
This field contains an optional boot filename or null-terminated string

23. DHCP Data Field
DHCP Data Field is often refered to as the Options Field
This field is used to expand the data that is included in the DHCP packet.
This field always starts with the “Magic Cookie” number (0x ) and ends with the “End” identifier (0xFF)
DHCP Option 53: Message Type - This required option indicates the general purpose of any DHCP message

24. DHCP Options Field
The DHCP boot sequence uses the following message types:
DHCP Message Type 1: Discover (client to server)
DHCP Message Type 2: Offer (server to client)
DHCP Message Type 3: Request (client to server)
DHCP Message Type 4: ACK (server to client)

25. Summary
This presentation covered information that allowed the student to:
Describe the history of DHCP
Describe the DHCP address discovery process
Describe the fields contained in the DHCP header and their functions.