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APIPA: Automatic Private IP Addressing

What is APIPA?

APIPA stands for Automatic Private IP Addressing. It is a word used by Microsoft to describe address auto configuration, which enables a Local Area Network (LAN) computer to assign itself a distinctive IP address when Dynamic Host Configuration Protocol (DHCP) is not available. Employing the network management protocol known as DHCP, businesses can dynamically allocate IP addresses and other network settings to devices or nodes on their networks. The necessity to manually assign IP addresses to the controlled devices is removed by DHCP-based services, which automatically manage client network configurations from a central server.

Small businesses to large enterprises all can use DHCP to support their various network sizes. When DHCP servers are down, DHCP clients can still get IP addresses thanks to APIPA. Except for Windows NT, APIPA is present in all current versions of Windows. Unless the Registry of the computer is changed to disable it, it is enabled on all Windows DHCP clients. Additionally, network adapters can be configured to support APIPA. It is also known as Auto-IP, Auto-Net, or DHCP failover method (for local networks).

Since Windows 98 and Windows ME, Dynamic Host Configuration Protocol clients have had access to Automatic Private IP Addressing (APIPA), which allows them to dynamically assign IP addresses to themselves in the absence of a DHCP server. APIPA facilitates the configuration and maintenance of small LANs and acts as a DHCP server failover mechanism.

History of APIPA

Initially, the IPv4 address block 169.254.0.0/16 (169.254.0.0 - 169.254.255.255) has been set aside by the Internet Engineering Task Force (IETF) for link-local addressing. Traffic overload increases because of the simultaneous use of IPv4 addresses with various scopes. The link-local addresses are assigned to interfaces and are stateless in nature, allowing for the establishment of connection even in the absence of a response from the DHCP server. The address autoconfiguration technique is thus referred to by Microsoft as "Automatic Private IP Addressing (APIPA)".

How does APIPA work?

A DHCP client computer tries to contact a DHCP server whenever it connects to the network. If a server is available, it will give the DHCP client an IP address and other network information. A DHCP server might not be available in some circumstances. For instance, the server can be momentarily unavailable, or the network might not have any DHCP servers.

When this happens and APIPA is turned on, the client computer chooses an IP address from a list of predetermined addresses (ranging from 169.254.0.0 to 169.254.255.255) and assigns it to itself automatically. To avoid conflicts with DHCP routable addresses, the Internet Assigned Numbers Authority (IANA) allocates these addresses expressly for APIPA use cases.

The DHCP client employs the Address Resolution Protocol (ARP) as part of the APIPA assignment procedure to make sure the selected address isn't already in use by another network computer. Once the client has issued itself an IP address, it can communicate with other computers on the LAN using TCP/IP with a subnet mask of 255.255.0.0 that are either manually configured for APIPA or with the correct address range.

Advantages of APIPA

  • Even if the DCHP client has previously got an IP address from the DHCP server, using the APIPA service can be advantageous.
  • This is accurate even if the lease at that address has already run out.
  • A DHCP client usually notifies the user when it switches between DHCP and APIPA addressing.
  • In some circumstances, depending on the OS and how the DHCP client is set up, APIPA can be disabled even though it is enabled by default.
  • Because APIPA may allocate IP addresses to networking hosts if DHCP fails, it can be used as a backup for DHCP.
  • Inappropriate broadcasting is stopped.
  • ARP (Address Resolution Protocol) is used to verify that the address isn't already in use.

Disadvantages of APIPA

  • The network can be slowed down by APIPA IP addresses.
  • Unlike DHCP, APIPA does not have a network gateway.
  • Use of APIPA addresses in local area networks is restricted.
  • Devices with APIPA configuration adhere to the peer-to-peer communication rule.
  • It adheres to the peer-to-peer communication norm and only allows usage of the addresses within the local area network.

Characteristics of APIPA

  • In the event of a DHCP server failure, APIPA establishes correct connectivity.
  • The primary DHCP server's response and status are controlled by APIPA for a predetermined amount of time.