Symptom
DHCP clients have a Time Left value of "0 minutes" listed in Server Admin. This appears in the DHCP Clients window (Figure 1), which is accessed by choosing View DHCP Clients from the DHCP/NetBoot module of Server Admin.
Figure 1 The DHCP Clients window
Solution
Client lease expiration occurs at the time specified in the Configure DHCP window of Server Admin. When client leases expire, the DHCP Clients window of Server Admin shows a Time Left value of "0 minutes". These are leases that have expired and are available for reassignment, if necessary.
The DHCP server attempts to associate client hardware addresses with the same IP addresses whenever possible, so an expired lease is retained for future reassignment to the same hardware address that originally requested it. The DHCP server will recycle a leased IP address when these conditions are met:
The available IP address range (and thus maximum number of leases) is set in Server Admin. To access these settings, follow these steps:
These settings appear in the General pane of the subnet's individual configuration window.
Example
If you create a pool of 100 DHCP client addresses but only start up 25 computers, then the same 25 leases should always be given to those computers. Next, let these leases expire and start up 25 different computers. These clients now receive 25 new leases while the original 25 show as expired. If you start up the original 25 computers, they should get the same IP addresses they had before.
If you keep starting up new computers until 100 leases are used, each computer receives its leased IP address. At this point, no additional computers should be able to receive a new lease. If you shut down all computers and allow the leases to expire, a newly added system (computer 101) gets a new lease, but consumes a previously used DHCP IP address. Server Admin would indicated that there are now 99 leases with a Time Left value of 0 minutes.
DHCP clients have a Time Left value of "0 minutes" listed in Server Admin. This appears in the DHCP Clients window (Figure 1), which is accessed by choosing View DHCP Clients from the DHCP/NetBoot module of Server Admin.
Figure 1 The DHCP Clients window
Solution
Client lease expiration occurs at the time specified in the Configure DHCP window of Server Admin. When client leases expire, the DHCP Clients window of Server Admin shows a Time Left value of "0 minutes". These are leases that have expired and are available for reassignment, if necessary.
The DHCP server attempts to associate client hardware addresses with the same IP addresses whenever possible, so an expired lease is retained for future reassignment to the same hardware address that originally requested it. The DHCP server will recycle a leased IP address when these conditions are met:
- A number of client hardware addresses equal to or greater than the maximum number of leases has previously requested a lease.
- The number of computers with unexpired leases is less than the maximum number of leases.
- The lease of the IP address to be recycled is expired.
- The expired lease to be reassigned is associated with the least recently requesting hardware address.
The available IP address range (and thus maximum number of leases) is set in Server Admin. To access these settings, follow these steps:
- 1. Open Server Admin.
2. Click the Network tab.
3. Choose Configure DHCP from the DHCP/NetBoot module.
4. Click the Subnets tab, if necessary.
5. Select the subnet.
6. Click Edit.
These settings appear in the General pane of the subnet's individual configuration window.
Example
If you create a pool of 100 DHCP client addresses but only start up 25 computers, then the same 25 leases should always be given to those computers. Next, let these leases expire and start up 25 different computers. These clients now receive 25 new leases while the original 25 show as expired. If you start up the original 25 computers, they should get the same IP addresses they had before.
If you keep starting up new computers until 100 leases are used, each computer receives its leased IP address. At this point, no additional computers should be able to receive a new lease. If you shut down all computers and allow the leases to expire, a newly added system (computer 101) gets a new lease, but consumes a previously used DHCP IP address. Server Admin would indicated that there are now 99 leases with a Time Left value of 0 minutes.