GS-5220-Series User Manual

User’s Manual of GS-5220 Ultra PoE & PoE+ Series
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DHCP used by networked computers (clients) to obtain IP addresses and other parameters such as the default
gateway, subnet mask, and IP addresses of DNS servers from a DHCP server.
The DHCP server ensures that all IP addresses are unique, for example, no IP address is assigned to a second client
while the first client's assignment is valid (its lease has not expired). Therefore, IP address pool management is done
by the server and not by a human network administrator.
Dynamic addressing simplifies network administration because the software keeps track of IP addresses rather than
requiring an administrator to manage the task. This means that a new computer can be added to a network without the
hassle of manually assigning it a unique IP address.
DHCP Relay
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 accounts (banking, etc.), or other services that
rely on the affected computer.