Leases


Last Updated: 2021-08-31

When DHCP sends configuration information to a client, the information is
sent with a lease time. This is the length of time that the client can use
the IP address it has been assigned. The duration of the lease time can be
changed according to your specific requirement.

During the lease time, the DHCP server cannot assign that IP address to any
other clients. The purpose of a lease is to limit the length of time that a
client can use an IP address. A lease prevents unused clients from taking up
IP addresses when there are more clients than addresses. It also enables the
administrator to make configuration changes to all of the clients on the
network in a limited amount of time. When the lease expires, the client will
request a new lease from DHCP. If the configuration data has changed, the
new data will be sent to the client at that time.


Lease renewal


The client starts to renew a lease when half of the lease time has passed.
For example, for a 24-hour lease, the client will attempt to renew the lease
after 12 hours. The client requests the renewal by sending a DHCPREQUEST
message to the server. The renewal request contains the current IP address
and configuration information of the client.

If the server accepts the request, it will send an DHCPACK message back to
the client. If the server does not respond to the request, the client can
continue to use the IP address and configuration information until the lease
expires. If the lease is still active, the client and server do not need to
go through the DHCPDISCOVER and DHCPREQUEST process. When the lease has
expired, the client must start over with the DHCPDISCOVER process.

If the server is unreachable, the client can continue to use the assigned
address until the lease expires. In the previous example, the client has 12
hours from when it first tries to renew the lease until the lease expires.
During a 12-hour outage, new users cannot get new leases, but no leases will
expire for any computer turned on at the time that the outage starts.


Determining lease duration


The default lease time for the DHCP server is 24 hours. When setting the
lease time on your DHCP server, consider your goals, usage patterns of your
site, and service arrangements for your DHCP server. Use the following
questions to help you decide on an appropriate lease time.


Do you have more users than addresses? 
If so, the lease time must be short so that clients do not need to wait
for unused leases to expire.


Do you have a minimum amount of time that you need to support? 
If your typical user is on for an hour at minimum, that suggests an hour
lease at minimum.


How much DHCP message traffic can your network handle? 
If you have a large number of clients or slow communication lines over
which the DHCP packets will run, network traffic might cause problems. The
shorter the lease, the heavier traffic the server and network load from the
renewal request on your network.


What kind of service plan do you have in place 
and to what extent can your network handle an outage?
Consider any routine maintenance, and the potential impact of an outage.
If the lease time is at least twice the server outage time, then running
clients who already have leases will not lose them. If you have a good idea
of your longest likely server outage, you can avoid such problems.


What type of network environment is the DHCP server in? 
What does a typical client do?

Consider what the clients do on the network that the DHCP server is
servicing. For example, if you have an environment where the clients are
primarily mobile, connecting to the network at varying times, and checking
their e-mail typically only once or twice a day, you might want a relatively
short lease time. In this case, it might not be necessary to have a single
IP address set aside for every client. By limiting the lease time, you can
use fewer IP addresses to support the mobile clients.

Alternatively, if you have an office environment where most of the
employees have primary workstations in a fixed location, a lease time of 24
hours might be more appropriate. It might also be necessary in this
environment to have an IP address available for each client that connects to
the network during business hours. In this case, if you specify a shorter
lease time, the DHCP server negotiates the lease renewal much more
frequently with the clients, which causes excess network traffic.


How much does your network configuration change? 
If your network topology changes quite frequently, you might want to
stay away from longer leases. Long leases can be disadvantageous in cases
where you need to change a configuration parameter. The length of the lease
can mean the difference between having to go to every affected client and
restarting it, or merely waiting a certain amount of time for the leases to
be renewed.

If your network topology rarely changes and you have enough IP addresses
in your address pool, you can configure DHCP to use infinite leases, that
is, leases that never expire. However, infinite leases are not recommended.
If you use an infinite lease, the IP address is leased to the client
indefinitely. These clients do not need to go through any lease renewal
process after they receive the infinite lease. After an infinite lease is
assigned to a client, that address cannot be assigned to another client.
Therefore, if you want to assign that client a new IP address or lease the
client's IP address to another client later, problems might occur.
You might have clients in your network, such as a file server, that will
always receive the same IP address. Rather than using an infinite lease,
assign a specific address to the client and give it a long lease time. The
client still must lease it for a given amount of time and renew the lease,
but the DHCP server reserves the IP address for that client only. Then, if
you get a new file server, for example, you can just change the client
identifier (MAC address) and the DHCP server gives the new file server that
same address. If you have given it an infinite lease, then the DHCP server
cannot give out the address again unless the lease is explicitly deleted.