NA Lesson - 4

Automatic Private IP Addressing (APIPA )

• APIPA is an addressing feature for simple networks that consist of a single network segment.
• Whenever a computer running Windows Server 2003 has been configured to obtain an IP address automatically, and when no DHCP server or alternate configuration is available, the computer uses APIPA to assign itself a private IP address in the range of 169.254.0.1–169.254.255.254.
•  To determine whether Automatic Private IP Addressing is currently enabled and active, enter ipconfig /all at a command prompt.
• The resulting text identifies your IP address and other information. If the Autoconfiguration Enabled line reads Yes and the IP address is in the 169.254.0.1–169.254.255.254 range, Automatic Private IP Addressing is active.
• This automatic addressing feature works only for computers on a network segment that cannot obtain an IP address through other means.
• If a DHCP server later becomes available to a host that has assigned itself an APIPA address, the computer changes its IP address to one obtained from the DHCP server.
• Computers using APIPA addresses can communicate only with other computers using APIPA addresses on the same network segment; they are not directly reachable from the Internet.
• We should note also that through APIPA, we cannot configure a computer with a DNS server address, a default gateway address, or a WINS server address.

Disabling APIPA

•  If we want to ensure that APIPA will not be used, we can either configure an alternate address in the connection’s IP properties, or disable the automatic addressing feature by editing the Registry.
• Note that to disable APIPA for one adapter and to disable APIPA for all adapters requires you to edit different Registry keys.
• To disable APIPA on a single adapter by editing the Registry, complete the following steps:

1. Use the Registry Editor Regedit.exe to add the Registry entry        IPAutoconfigurationEnabled with a value of 0 (REG_DWORD data type) in the following subkey:

    HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters\Interfaces\interface

2. Restart the computer.

• To disable APIPA for multiple adapters by editing the Registry, complete the following steps:

1. Set the value of the IPAutoconfigurationEnabled entry to 0 (REG_DWORD data type) in the following Registry subkey:

            HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Tcpip\Parameters

2. Restart the computer.

Troubleshooting APIPA

• For computers running any version of Windows since Windows 98, APIPA addresses are default addresses.
• That is, they are assigned to connected hosts whose network configuration has not been altered since the operating system was installed. In certain small networks, you might want to leave the computers with these default APIPA addresses so as to simplify network communication and administration.
• We can run the Ipconfig /all command on networked computers to determine whether the address assigned to each computer’s local area connection falls within the APIPA range of 169.254.0.1–169.254.255.254.
•  If the Ipconfig /all command does not reveal an APIPA address, the output instead reveals one of three scenarios:

            (i) no address with or without an error message,

            (ii)an all zeros address

            (iii)a nonzero IP address outside of APIPA range.