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The TCP/IP Guide  9  TCP/IP Application Layer Protocols, Services and Applications (OSI Layers 5, 6 and 7)       9  TCP/IP Network Configuration and Management Protocols (BOOTP, DHCP, SNMP and RMON)            9  Host Configuration and TCP/IP Host Configuration Protocols (BOOTP and DHCP)                 9  TCP/IP Dynamic Host Configuration Protocol (DHCP)                      9  DHCP Address Assignment and Dynamic Address Allocation and Management

DHCP Address Assignment and Allocation Mechanisms 1 2 3 4 DHCP Lease "Life Cycle" Overview (Allocation, Reallocation, Renewal, Rebinding and Release) and Lease Timers

DHCP Lease Length Policy and Choosing Lease Durations

When dynamic address allocation is used, the administrator of the network must provide parameters to the DHCP server to control how leases are assigned and managed. One of the most important decisions to be made is the lease length policy of the internetwork: how long the administrator wants client leases to last. There of course is no “right answer”—the right lease length interval depends on the network, the server, and the clients. The choice of lease time, like so many other networking parameters, boils down to a trade-off between stability and allocation efficiency.

The primary benefit of using long lease times is that the addresses of devices are relatively stable. A device doesn't have to worry about its IP address changing all the time—and neither does its user. This is a significant advantage in many cases, especially when it is necessary for the client to perform certain server functions, accept incoming connections, or use a DNS domain name (ignoring for the moment dynamic DNS capabilities). In those situations, having the IP address of a device moving all over the place can cause serious complications.

The main drawback of using long leases is that they substantially increase the amount of time that an IP address, once no longer needed, is “tied up” before it can be reused. In the worst case scenario, the amount of “wasted time” for an allocation can be almost as long as the lease itself. If we give a device a particular address for six months and after two weeks the device is shut down and no longer used, the IP address that it was using is still “unavailable” for another five and a half more months.

For this reason, many administrators prefer to use short leases. This forces a client to continually renew the lease as long as it needs it. When it stops asking for permission again, the address is quickly put back into the pool. This makes shorter leases a better idea in environments where the number of addresses is limited and must be conserved. The drawback, of course, is the opposite of the benefit of long leases: constantly-changing IP addresses.

DHCP Address Assignment and Allocation Mechanisms 1 2 3 4 DHCP Lease "Life Cycle" Overview (Allocation, Reallocation, Renewal, Rebinding and Release) and Lease Timers