10 DHCP Lease Time Best Practices

DHCP lease time is an important setting that can impact network performance and security. A DHCP server can be configured to give out leases for a specific amount of time, and it’s important to choose a lease time that is appropriate for your network.

In this article, we will discuss 10 DHCP lease time best practices that you can use to optimize your network.

1. Use the default lease time

The default lease time is set to 86400 seconds, or one day. This is the amount of time that a DHCP server will keep an IP address assigned to a client before it expires and is available for another client to use.

While it’s possible to change the DHCP lease time, it’s not recommended. The reason for this is that changing the DHCP lease time can cause problems with clients that are configured to use a static IP address.

If the DHCP lease time is shorter than the static IP address lease time, then the client will lose its IP address when the DHCP lease expires. If the DHCP lease time is longer than the static IP address lease time, then the client will keep its IP address even after it’s no longer using it, which can cause problems with other clients trying to use that IP address.

For these reasons, it’s best to leave the DHCP lease time at the default value.

2. Set a maximum lease time

If a DHCP server doesn’t have a maximum lease time set, then it will hand out IP addresses to clients indefinitely. This can cause problems because it means that the same IP address could be assigned to multiple devices over time, which can lead to conflicts.

It’s generally best to set a maximum lease time of around 24 hours, which should be plenty of time for most devices. If you need to support devices that require a longer lease time, then you can always create a separate scope with a longer lease time for those devices.

3. Consider setting a minimum lease time

When a DHCP client’s lease expires, it will send a DHCPRELEASE message to the DHCP server. If the client doesn’t receive a DHCPACK in response, it will assume that the DHCP server is unavailable and will start using a link-local address.

If the client does receive a DHCPACK, it will renew its lease and continue using the assigned IP address. However, if the minimum lease time has expired, the client will request a new IP address from the DHCP server.

This process can cause problems because the client will keep requesting a new IP address until it receives one. This can lead to network congestion and decreased performance.

By setting a minimum lease time, you can prevent this problem from happening. The minimum lease time is the amount of time that must elapse before the client can request a new IP address.

For example, if you set the minimum lease time to 12 hours, the client will not be able to request a new IP address for at least 12 hours. This will give the DHCP server time to process the release and renew the lease.

4. Monitor DHCP leases

If you’re not monitoring DHCP leases, you won’t be able to tell when a device has been assigned an IP address that it shouldn’t have. This could lead to all sorts of problems, such as security breaches and network outages.

Fortunately, there are a few different ways to monitor DHCP leases. One is to use a tool like SolarWinds® DHCP Scope Monitor, which allows you to see which IP addresses have been leased, for how long, and to which devices.

Another way to monitor DHCP leases is to use the native logging features of your DHCP server. This can be a bit more complicated, but it’s still possible to get the information you need.

Either way, it’s important to make sure that you’re monitoring DHCP leases so that you can spot potential problems before they cause serious damage.

5. Don’t use static IP addresses for servers

If you use static IP addresses for servers, then every time the DHCP server’s lease time expires, the server will need to be reconfigured with a new static IP address. This is not only time-consuming, but it can also lead to errors.

It’s much easier and less error-prone to simply use DHCP reservations for servers. With DHCP reservations, you can set aside a specific IP address for a server, and as long as the DHCP server’s lease time is renewed before it expires, the server will always have the same IP address.

6. Ensure that your DHCP server is secure

If an attacker were to gain access to your DHCP server, they could easily spoof DHCP responses and redirect traffic to a malicious site. This type of attack is known as a man-in-the-middle attack, and it can be devastating to your network.

To prevent this from happening, make sure that your DHCP server is properly configured and that only authorized devices have access to it. You should also consider using a firewall to further protect your DHCP server.

7. Use multiple scopes when needed

When you have a single DHCP server and multiple scopes, each scope can have its own lease time. This is important because it allows you to fine-tune the lease time for each individual scope.

For example, you may want to have a shorter lease time for your wireless network than your wired network. Or, you may want to have a longer lease time for your desktop computers than your laptops.

By using multiple scopes, you can make sure that each device on your network gets the appropriate lease time. This will help to ensure that your devices are always able to connect to the network and stay connected.

8. Limit the number of DHCP clients per subnet

If you have too many DHCP clients on a single subnet, it can lead to network performance issues. This is because each DHCP client will send out a DHCP Discover packet every time it wants to renew its lease, and if there are too many clients doing this at the same time, it can cause a flood of traffic.

To avoid this issue, make sure to limit the number of DHCP clients per subnet. You can do this by using DHCP reservations, which will assign a specific IP address to a particular DHCP client. This way, you can ensure that each client has its own IP address and won’t be competing with other clients for leases.

9. Reserve IP addresses for specific devices

When a DHCP server leases an IP address to a client, it records the MAC address of the client’s network adapter and the lease time. If the client renews the lease before it expires, the DHCP server updates the lease time.

However, if the client doesn’t renew the lease before it expires, the DHCP server assumes that the client is no longer using the IP address and can lease it to another client.

If you have a device that needs a static IP address, such as a server, you should reserve an IP address for that device. That way, even if the device is turned off or disconnected from the network, the DHCP server won’t lease the IP address to another client.

10. Avoid using DHCP on WAN links

If you use DHCP on a WAN link, and the link goes down, when it comes back up, the DHCP server will likely have given out the same IP address to another device. This will cause problems because both devices will now have the same IP address.

To avoid this problem, you should statically configure the IP addresses on devices that are connected to WAN links.