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Figure 2: Description of fields in a DHCP message

Field Octets Description
op 1 Message op code / message type. 1 = BOOTREQUEST, 2 = BOOTREPLY
htype 1 Hardware address type,, see ARP section in "Assigned Numbers" RFC; e.g.,`1' = 10mb ethernet.
hlen 1 Hardware address length (e.g., `6' for 10mb ethernet).
hops 1 Client sets to zero, optionally used by relay-agents when booting via a relay-agent.
xid 4 Transaction ID, a random number chosen by the client, used by the client and server to associate messages and responses between a client and a server.
secs 2 Filled in by client, seconds elapsed since client started trying to boot.
flags 2 Flags.
ciaddr 4 Client IP address; filled in by client in DHCPREQUEST if verifying previously allocated configuration parameters.
yiaddr 4 `your' (client) IP address.
siaddr 4 IP address of next server to use in bootstrap; returned in DHCPOFFER, DHCPACK, and DHCPNAK by server.
giaddr 4 Relay agent IP address,, used in booting via a relay-agent.
chaddr 16 Client hardware address.
sname 64 Optional server host name, null-terminated string.
file 128 Boot file name, null-terminated string; "generic" name or null in DHCPDISCOVER, fully qualified directory-path name in DHCPOFFER.
options 312 Optional parameters field.