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Review of APIPA (Automatic Private IP Addressing)

If you need to refresh your understanding of APIPA, read through the following material.
Windows supports a mechanism for automatic IP address assignment for simple LAN-based network configurations. This addressing mechanism, called Automatic Private IP Addressing (APIPA), is an extension of dynamic IP address assignment for LAN adapters, enabling the configuration of IP addresses without using static IP address assignment or installing the DHCP Server Service.
The following SlideShow describes how APIPA works for a client who does not already have an active DHCP lease:

APIPA - Automatic Private IP Addressing

(Automatic Private IP Addressing) The Windows function that provides DHCP autoconfiguration addressing. APIPA assigns a class B IP address from 169.254.0.0 to 169.254.255.255 to the client when a DHCP server is either permanently or temporarily unavailable. Designed for small non-routable networks, if a DHCP server becomes available later, the APIPA address is replaced with one from the DHCP server. For example, when a Windows Vista machine starts up, it waits only six seconds to find a DHCP server before assigning an IP from the APIPA range. It then continues to look for a DHCP server. Previous versions of Windows looked for a DHCP server for up to three minutes.

DHCP
Request for IP address: When Windows starts, TCP/IP attempts to find a DHCP server on the attached network to obtain a dynamically assigned IP address.


No IP address returned: In the absence of a DHCP server, the client cannot obtain an IP address.


APIPA generates an IP address in the form of 169.254.x.y (where x.y. is a unique identifier on the network that the client generates and a subnet mask of 255.255.0.0. After generating the address, the client computer issues a "gratuitous ARP broadcast", and then, if no other computer responds, assigns the address to itself.

The client computer will continue to search for a DHCP server by issuing a DHCP Discover message every five minutes. If a DHCP server does come online, the auto-configure DHCP client will receive information from the DHCP server and be able to participate in normal network activity. This allows the computer to be connected to a LAN hub, to restart without any IP address configuration and to use TCP/IP for local network access.

Configuring the LAN

For the APIPA feature to function properly on a computer running Windows 2000, you must configure a network LAN adapter for TCP/IP and click Obtain an IP address automatically in the Internet Protocol (TCP/IP) Properties dialog box.

Internet Protocol Properties dialog box

Open Source Intelligence
Microsoft has reserved IP addresses 169.254.0.1 through 169.254.255.254, and uses this range to support APIPA. Remember that these are classified as private IP address ranges, and therefore are not routable through the Internet.

Disabling APIPA

By default, the APIPA feature is enabled. However, you can disable this feature by adding the IPAutoconfigurationEnabled entry to the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services \Tcpip\Parameters\Interfaces\GUID_of_the_adapter subkey in the registry. The IPAutoconfigurationEnabled entry takes a REG_DWORD data type. To disable APIPA, specify a value of 0 for the entry.

Automatic IP Address Assignment for BOOTP Clients

Rather than completely eliminating BOOTP or providing and maintaining static address allocations for all BOOTP clients, you can have some DHCP servers automatically allocate IP addresses to BOOTP clients. This form of BOOTP service is sometimes called dynamic BOOTP.
Dynamic BOOTP is attractive because it enables you to serve BOOTP clients from the same pool of addresses as dynamic clients. It doesn’t require you to create DHCP reservations, yet it isn’t adversely affected if you do. This configuration has the added advantage of enabling BOOTP clients to function correctly even if they roam across multiple subnets or if the network is renumbered. The disadvantage of dynamic BOOTP is that the DHCP server must permanently reserve any IP addresses it assigns to BOOTP clients because BOOTP does not include any mechanism for automatically recovering IP addresses that are no longer in use.