E94AYCEN communication manual (Ethernet)
Commissioning
DHCP implementation in the Servo Drive 9400
36 L EDS94AYCEN EN 9.0 - 09/2012
6.3.4 DHCP packet structure
The DHCP packets have the following structure:
Description of the fields:
Bit 1 ... 8 Bit 9 ... 16 Bit 17 ... 24 Bit 25 ... 32
op (1 byte) htype (1 byte) hlen (1 byte) hops (1 byte)
xid (4 bytes)
secs (2 bytes) flags (2 bytes)
ciaddr (4 bytes)
yiaddr (4 bytes)
siaddr (4 bytes)
giaddr (4 bytes)
chaddr (4 bytes)
sname (4 bytes)
file (4 bytes)
options (variable)
Field Size Description
op 1 Byte Opcode: Task carried out by the DHCP packet
• Indicates a client request or a server response.
htype 1 Byte Hardware type: Specification of the network topology
• Examples: 1 for Ethernet, 15 for Frame Relay (specification in RFC 1700)
hlen 1 Byte Hardware address length: Length of the hardware address in the "chaddr"
(client hardware address) field
hops 1 Byte Hop count: Number of routers / gateways between the client and the server
xid 4 bytes Transaction ID: Unique identifier generated by the client
• This is required in order to assign a DHCP response to the corresponding
DHCP request.
secs 2 bytes Number of seconds: Time in seconds that has elapsed since the DHCP
process began
flags 2 bytes Flags: The first bit is used as a broadcast flag. All other flags are reserved for
later use (status = 0).
ciaddr 4 bytes Client IP address: Most recently used client IP address
• This is only used in a client DHCP request.
yiadr 4 bytes Your IP address: IP address assigned to the client by the server
• This is only used in a server DHCP response.
siaddr 4 bytes Server IP address: IP address of the server
• This is only used in a server DHCP response.
giaddr 4 bytes Gateway IP address: This field enables the client to communicate with
servers in other DHCP subnetworks.
• IP address "0.0.0.0" in a client request
• A DHCP relay agent enters its IP address here.
chaddr 4 bytes Client hardware address: MAC address of the client
sname 4 bytes Server host name: This field is optional and can contain the server name.
file 4 bytes Boot filename: The client defines the full path for its boot file here.
options Variable
(1 to 4 bytes)
Options: This field contains additional information for the client.
• The specification of the DHCP message type, for example, is very
important.
• Defined in full in RFC 2132
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