Tag Archives: packet-format

DHCP format

Table 1 BOOTP Request and Reply Format
Field Bytes Name Description
op 1 OpCode Identifies the packet as a request or reply. 1=BOOTREQUEST and 2=BOOTREPLY.
htype 1 Hardware Type Specifies the network hardware type.
hlen 1 Hardware Length Specifies the length hardware address length.
hops 1 Hops The client sets the value to zero and the value increments if the request is forwarded across a router.
xid 4 Transaction ID A random number that is chosen by the client. All DHCP messages exchanged for a given DHCP transaction use the ID (xid).
secs 2 Seconds Specifies number of seconds since the DHCP process started.
flags 2 Flags Indicates whether the message will be broadcast or unicast.
ciaddr 4 Client IP Address Used when the client is aware of the IP address as in the case of the Bound, Renew, or Rebinding states.
yiaddr 4 Your IP Address If the client IP address is, the DHCP server places the offered client IP address in this field.
siaddr 4 Server IP Address If the client knows the IP address of the DHCP server, this field is populated with the DHCP server address. Otherwise, it is used in DHCPOFFER and DHCPACK from the DHCP server.
giaddr 4 Router IP Address The gateway IP address, filled in by the DHCP/BootP Relay Agent.
chaddr 16 Client MAC Address The DHCP client MAC address.
sname 64 Server Name The optional server hostname.
File 128 Boot Filename The boot filename.
Options Variable Option Parameters The optional parameters that can be provided by the DHCP server. RFC 2132 lists all possible options.