124 CHAPTER 9: DHCP OVERVIEW
DHCP IP Address Assignment
This section contains information on DHCP IP Address Assignments.
IP Address Assignment
Policy
Currently, DHCP provides the following three IP address assignment policies to meet
the requirements of different clients:
■Manual assignment. The administrator statically binds IP addresses to the few
clients with special uses (such as WWW server). Then the DHCP server assigns
these fixed IP addresses to the clients.
■Automatic assignment. The DHCP server assigns IP addresses to DHCP clients
when they first connect to the network. The IP addresses will be occupied by the
DHCP clients permanently.
■Dynamic assignment. The DHCP server leases IP addresses for predetermined
period of time and reclaims them at the expiration of the period. In this case, a
DHCP client must apply for an IP address regularly. This policy is suitable for
temporarily IP address requests and is applied to most clients.
DHCP IP Address
Preferences
The order in which a DHCP server assigns IP addresses to DHCP clients are as follows:
■IP addresses that are statically bound to the MAC addresses of DHCP clients
■IP addresses that are reclaimed by DHCP servers. That is, those in the Option fields
of DHCP-REQUEST packets sent by DHCP clients
■The IP addresses in the DHCP address pool
■IP addresses that are expired or conflict
Sending Device
Information through
DHCP Option60
As a DHCP extended option defined in RFC2132, Option60 is used to send the device
information of a client through a DHCP request packet. When requesting for an IP
address, a DHCP client adds the manufacturer name, product type, and other
information of this device to the Option60 field of a DHCP request packet and sends
the packet to the DHCP server. You cannot change this device information, which is
controlled by host software.
Table109 lists the device information provided by 5500 series Ethernet switches
through DHCP requests.
Table109 Device information that 5500 series Ethernet switches add to DHCP Option60
Model Device information in Option60
5500-SI 28-Port 3Com-Switch-5500-SI
5500-SI 52-Port 3Com-Switch-5500-SI
5500-EI 28-Port 3Com-Switch-5500-EI
5500-EI 52-Port 3Com-Switch-5500-EI
5500-EI PWR 28-Port 3Com-Switch-5500-EI
5500-EI PWR 52-Port 3Com-Switch-5500-EI
5500-EI 28-Port FX 3Com-Switch-5500-EI
5500G-EI 24-Port 3Com-Switch-5500G-EI
5500G-EI 48-Port 3Com-Switch-5500G-EI
5500G-EI PWR 24-Port 3Com-Switch-5500G-EI
5500G-EI PWR 48-Port 3Com-Switch-5500G-EI
5500G-EI SFP 24-Port 3Com-Switch-5500G-EI