Dynamic Host Configuration Protocol Lease Durations

Windows Server 2008 creates a lease period of 8 days for wired networks and 6 hours for wireless networks. You can accept the default settings on networks that meet the following requirements:

■ Less than one-third of the available DHCP scope is in use at any one time.

■ Client computers are primarily desktops and remain connected to the network for more than a week at a time.

■ IP addresses of DNS servers, WINS servers, and routers are not changed regularly.

If a network does not meet any of these requirements, you might need to use a shorter lease period. For example, wireless networks have a default lease period of 6 hours because wireless computers tend to stay connected for a short period of time. Similarly, wired networks with a large number of mobile computers and remote access connections (such as a virtual private network) should have a shorter lease period because computers are likely to use an IP address for less than a day. If more than half your DHCP scope is in use during peak hours, a shorter lease period reduces the likelihood that the DHCP server will run out of available addresses.

Shorter lease periods allow you to change IP address settings in a shorter time frame. For example, if you are replacing your DNS server with a server that uses a new IP address, you can immediately update the options on the DHCP server. However, you will need to run both the old and the new DNS server during the period of time that DHCP clients retain their original IP settings. With a shorter DHCP lease of 6 hours, you can be assured that DHCP clients will have updated DNS server configuration information by the end of the lease period, allowing you to disconnect the old DNS server the following day. With an 8-day lease period, you would need to leave the old DNS server online for more than a week.

The disadvantage to shorter DHCP lease durations is increased network traffic for DHCP renewals. However, the bandwidth required by DHCP lease renewals in relation to the bandwidth of modern Local Area Networks (LANs) is insignificant. For example, with a relatively short lease period of 6 hours, only two small packets will be transmitted for each DHCP client every three hours. The amount of additional bandwidth required is hardly measurable and will have no impact on network performance. Therefore, you can use shorter DHCP lease durations with no significant penalty.