TP-Link T2600G-28TS User Manual page 206

unicast IP datagrams before its protocol software has been configured should clear the first bit
to 0. A server or relay agent sending or relaying a DHCP message directly to a DHCP client
should examine the first bit in the 'flags' field. If this bit is set to 1, the DHCP message should
be sent as an IP broadcast and if the bit is cleared to 0, the message should be sent as an IP
unicast. The remaining bits of the flags field are reserved for future use and must be set to
zero by clients and ignored by servers and relay agents.
8) ciaddr：Client IP address, filled in by client in DHCPREQUEST when verifying previously
allocated configuration parameters.
9) yiaddr：'your' (client) IP address, configuration parameters allocated to the client by DHCP
server.
10) siaddr：IP address of next server to use in bootstrap, returned in DHCPOFFER, DHCPACK
and DHCPNAK by server.
11) giaddr：Relay agent IP address, used in booting via a relay-agent.
12) chaddr：Client hardware address.
13) sname：Optional server host name, null terminated string.
14) file：Boot file name, null terminated string, "generic" name or null in DHCPDISCOVER, fully
qualified directory-path name in DHCPOFFER.
15) options： Optional parameters field. See the options documents (RFC 2132) for a list of defined
options. We will introduce some familiar options in the next section.
DHCP Option

This section defines a generalized use of the 'options' field for giving information useful to a wide
class of machines, operating systems and configurations. Sites with a single DHCP server that is
shared among heterogeneous clients may choose to define other, site-specific formats for the use
of the 'options' field. Figure 10-14 gives the format of options field.
All options begin with a Code octet, which uniquely identifies the option followed by the length
octet. The value of the length octet does not include the Code and Length octets. The common
options are illustrated as below.
1) option 1：Subnet Mask option. The subnet mask option is option1 which identifies the
assigned IP address with network, and its length is 4 octets.
2) option 3： Router option. The router option is option 3 which specifies an IP address for routers
on the client's subnet.
3) option 6：DNS option. The DNS option is option 6, and it assigns the IP address of domain
name server to the client which allows the client can use the web service in the internet.
4) option 12：Host Name option. The option12 is used to specify the name of the client, which
may be requested by the DHCP server for authentication.
5) option 50：Requested IP Address option. The option 50 is used in a DHCP-REQUEST
message to allow the client to request the particular IP address.
Figure 10-14 DHCP Option
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