APIPA

For a network client to have an IPv4 address you either need to

• configure IP address manually or
• selected Automatic IP address so as to get the IP address automatically from a DHCP Server.

If you have selected for Automatic IP address and if the client fails to get an IP address using DHCP (for some reason)

It can discover an address on its own using APIPA i.e Automatic Private IP Address which is a.k.a IPv4 link local address which allows the client to communicate
within other client/s in the same subnet who are assigned with APIPA address.

To get an IPv4 APIPA address the client will select an random address from the range

169.254.1.0 to 169.254.254.255 with the network mask of 255.255.0.0

This because IETF has assigned 169.254.0.0/16 block for APIPA out of which first 256  and last 256 address are reserved .

The client will then send an ARP packet asking for the MAC address that corresponds to the randomly generated IPV4 address.

If any other client is using the address the client will generate another random address and try again.

Also note that there will no default gateway for the APIPA

Since the APIPA ip address are private address and

APIPA client can communicate only with the client/s in the same network (connected to the same switch) (it wont able to communicate outside the network )

APIPA is implemented in Windows 98 and later & is available in classic Mac OS 8.5 through 9 and in macOS.


Thanks for Reading !!!!