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DHCP for Windows 2000 by Neall Alcott

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DHCP Packet Structure

Let’s examine the packet structure of the DHCP protocol. As you will see, the DHCP packet structure is fundamentally the same as the BOOTP packet structure. There are only two differences: the vend field was changed to the options field, and this field was expanded from 64 octets in length to a minimum of 312 octets.

As shown in Figure 3.1, a DHCP packet’s transmission order is from left to right, top to bottom. The number shown in parentheses is the number of octets (or bytes) each field occupies.

DHCP packet structure

Figure 3-1. DHCP packet structure

Field Definitions

Table 3.1 lists the available fields in a DHCP packet and describes the type of information each field supplies.

Table 3-1. DHCP Field Definitions

Field

Definition

op

Specifies the message type: if 1, the message is a BOOTPREQUEST; if 2, the message is a BOOTPREPLY.

htype

Specifies the hardware address type (i.e., ethernet or token ring). 10 Mbps Ethernet = 1.

hlen

Specifies the hardware address length. Ethernet = 6.

hops

Specifies the number of hops between the client and the server.

xid

Specifies the transaction ID number. This is a random number that is used to match up the request with the reply that is returned.

secs

Specifies how long it has been since the client was booted.

flags

Specifies whether the BOOTPREPLY message should be sent as a unicast message or a broadcast message. ...

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