Planet GS-5220 Series User Manual page 549

DHCP Relay is used to forward and to transfer DHCP messages between the clients and the
server when they are not on the same subnet domain.
The DHCP option 82 enables a DHCP relay agent to insert specific information into a DHCP
request packets when forwarding client DHCP packets to a DHCP server and remove the
specific information from a DHCP reply packets when forwarding server DHCP packets to a
DHCP client. The DHCP server can use this information to implement IP address or other
assignment policies. Specifically the option works by setting two sub-options: Circuit ID (option 1)
and Remote ID (option2). The Circuit ID sub-option is supposed to include information specific to
which circuit the request came in on. The Remote ID sub-option was designed to carry
information relating to the remote host end of the circuit.
The definition of Circuit ID in the switch is 4 bytes in length and the format is "vlan_id"
"module_id" "port_no". The parameter of "vlan_id" is the first two bytes represent the VLAN ID.
The parameter of "module_id" is the third byte for the module ID. The parameter of "port_no" is
the fourth byte and it means the port number.
The Remote ID is 6 bytes in length, and the value is equal the DHCP relay agents MAC address.
DHCP Snooping
DHCP Snooping is used to block intruder on the untrusted ports of the switch device when it tries
to intervene by injecting a bogus DHCP reply packet to a legitimate conversation between the
DHCP client and server.
DNS
DNS is an acronym for Domain Name System. It stores and associates many types of information with
domain names. Most importantly, DNS translates human-friendly domain names and computer
hostnames into computer-friendly IP addresses. For example, the domain name www.example.com
might translate to 192.168.0.1.
DoS
DoS is an acronym for Denial of Service. In a denial-of-service (DoS) attack, an attacker attempts to
prevent legitimate users from accessing information or services. By targeting at network sites or network
connection, an attacker may be able to prevent network users from accessing email, web sites, online
User's Manual of GS-5220 LCD Series
549