DHCP Address Allocation Methods

There are three ways that a DHCP server either assigns or sends an IP address to a client:
  • Automatic allocation—The DHCP server assigns a permanent IP address to a client from its IP Pools. On the firewall, a Lease specified as Unlimited means the allocation is permanent.
  • Dynamic allocation—The DHCP server assigns a reusable IP address from IP Pools of addresses to a client for a maximum period of time, known as a lease. This method of address allocation is useful when the customer has a limited number of IP addresses; they can be assigned to clients who need only temporary access to the network. See the DHCP Leases section.
  • Static allocation—The network administrator chooses the IP address to assign to the client and the DHCP server sends it to the client. A static DHCP allocation is permanent; it is done by configuring a DHCP server and choosing a Reserved Address to correspond to the MAC Address of the client device. The DHCP assignment remains in place even if the client logs off, reboots, has a power outage, etc.
    Static allocation of an IP address is useful, for example, if you have a printer on a LAN and you do not want its IP address to keep changing, because it is associated with a printer name through DNS. Another example is if a client device is used for something crucial and must keep the same IP address, even if the device is turned off, unplugged, rebooted, or a power outage occurs, etc.
    Keep these points in mind when configuring a Reserved Address:
    • It is an address from the IP Pools. You may configure multiple reserved addresses.
    • If you configure no Reserved Address, the clients of the server will receive new DHCP assignments from the pool when their leases expire or if they reboot, etc. (unless you specified that a Lease is Unlimited).
    • If you allocate all of the addresses in the IP Pools as a Reserved Address, there are no dynamic addresses free to assign to the next DHCP client requesting an address.
    • You may configure a Reserved Address without configuring a MAC Address. In this case, the DHCP server will not assign the Reserved Address to any device. You might reserve a few addresses from the pool and statically assign them to a fax and printer, for example, without using DHCP.

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