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DHCP Configuration
108
Figure 15-3 DHCP packet format
The field meanings are illustrated as follows:
op: Operation types of DHCP packets: 1 for request packets and 2 for response packets.
htype, hlen: Hardware address type and length of the DHCP client.
hops: Number of DHCP relays which a DHCP packet passes. For each DHCP relay that
the DHCP request packet passes, the field value increases by 1.
xid: Random number that the client selects when it initiates a request. The number is used
to identify an address-requesting process.
secs: Elapsed time after the DHCP client initiates a DHCP request.
flags: The first bit is the broadcast response flag bit. It is used to identify that the DHCP
response packet is sent in the unicast or broadcast mode. Other bits are reserved.
ciaddr: IP address of a DHCP client.
yiaddr: IP address that the DHCP server assigns to a client.
siaddr: IP address of the DHCP server.
giaddr: IP address of the first DHCP relay that the DHCP client passes after it sent the
request packet.
chaddr: Hardware address of the DHCP client.
sname: Name of the DHCP server.
file: Name of the start configuration file that the DHCP server specifies for the DHCP client.
option: Optional variable-length fields, including packet type, valid lease time, IP address
of a DNS server, and IP address of the WINS server.
15.3 DHCP Relay
15.3.1 Usage of DHCP Relay
Since the packets are broadcasted in the process of obtaining IP addresses, DHCP is only
applicable to the situation that DHCP clients and DHCP servers are in the same network