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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