3Com 4500 PWR 26-Port Configuration Manual page 461

To improve security and avoid malicious attack to the unused sockets, S4500 Ethernet switches
provide the following functions:
UDP 67 and UDP 68 ports used by DHCP are enabled only when DHCP is enabled.
UDP 67 and UDP 68 ports are disabled when DHCP is disabled.
The corresponding implementation is as follows:
After a DHCP address pool is created by executing the dhcp server ip-pool command, the UDP 67
and UDP 68 ports used by DHCP are enabled.
After a DHCP address pool is deleted by executing the undo dhcp server ip-pool command and all
other DHCP functions are disabled, UDP 67 and UDP 68 ports used by DHCP are disabled
accordingly.
Configuring the dynamic IP address allocation mode
IP addresses dynamically assigned to DHCP clients (including those that are permanently leased and
those that are temporarily leased) belong to addresses segments that are previously specified.
Currently, an address pool can contain only one address segment, whose ranges are determined by
the subnet mask.
To avoid address conflicts, the DHCP server automatically excludes IP addresses (used by the gateway,
FTP server and so forth) specified with the dhcp server forbidden-ip command from dynamic
allocation.
The lease time can differ with address pools. But that of the IP addresses of the same address pool are
the same. Lease time is not inherited, that is to say, the lease time of a child address pool is not affected
by the configuration of the parent address pool.
Follow these steps to configure the dynamic IP address allocation mode:
To do...
Enter system view
Enter DHCP address pool view
Set the IP address segment
whose IP address are to be
assigned dynamically
Configure the lease time
Return to system view
Specify the IP addresses that
are not dynamically assigned
Use the command...
system-view
dhcp server ip-pool
pool-name
network network-address
[ mask mask ]
expired { day day [ hour hour
[ minute minute ] ] | unlimited }
quit
dhcp server forbidden-ip
low-ip-address
[ high-ip-address ]
39-8
Remarks
—
—
Required
By default, no IP address
segment is set. That is, no IP
address is available for being
assigned
Optional
The default lease time is one
day
—
Optional
By default, except the IP
addresses of DHCP server
interfaces, all IP addresses in a
DHCP address pool are
assignable.