Like DHCP, BOOTP is a broadcast-based system. Therefore, routers must be configured to forward BOOTP broadcasts. Today, it is far more likely that DHCP, rather than BOOTP, is used.
Automatic Private IP addressing (APIPA) is a feature introduced with Windows 98, and has been included in all subsequent Windows versions. The function of APIPA is that a system is capable of providing itself with an IP address in the event that it is incapable of receiving an address dynamically from a DHCP server. In such an event, APIPA assigns the system an address from the
169.254.0.0 address range and configures an appropriate subnet mask (
255.255.0.0). However, it doesn't configure the system with a default gateway address. As a result, communication is limited to the local network.
The idea behind APIPA is that systems on a segment can communicate with each other in the event of DHCP server failure. In reality, the limited usability of APIPA makes it little more than a last resort measure. For example, imagine that a system is powered on while the DHCP server is operational and receives an IP address of
192.168.100.2. Then the DHCP server fails. Now, if the other systems on the segment are powered on and are unable to get an address from the DHCP server because it is down, they would self-assign addresses in the
169.254.0.0 address range via APIPA. The systems with APIPA addresses would be able to talk to each other, but they couldn't talk to a system that received an address from the DHCP server. Likewise, any system that received an IP address via DHCP would be unable to talk to systems with APIPA assigned addresses. This, and the absence of a default gateway, is why APIPA is of limited use in real-world environments.