ARP Address Resolution Protocol

ARP (Address Resolution Protocol) is a low-level TCP/IP standard that works at the data-link layerand operates at level 2 of the OSI model.

It is defined by RFC 825.

It resolves the logical IP (Internet Protocol) address to the physical MAC (Media Access Control) address.

It broadcasts the request for the MAC address of all hosts across the network through the ethernet cable.

An ARP Table listing all IP and MAC addresses is maintained my most systems.

APIPA Automatic Private Internet Protocol Addressing

• APIPA (Automatic Private Internet Protocol Addressing) is alternatively known as zero configuration networking or address autoconfiguration.
• It is a TCP/IP standard designed by Microsoft as substitute to DHCP (Dynamic Host Configuration Protocol).
• It is used to automatically configure IP-based hosts when a DHCP Server is unavailable.
• APIPA is a useful solution for computer connected to a small local network (LAN) where all devices are connected on a single physical link.
• Computers that have been configured by APIPA have an IP address that is in the 169.254.0.0 range and have a subnet mask of 255.255.0.0.
• IETF's formal standard for APIPA functionality is RFC 3927 (Dynamic Configuration of IPv4 Link-Local Addresses.
• APIPA is useful in cases where there is no DHCP server and and there is either no previous IP address or the IP address's lease has expired.
• A Windows computer will retain the previous IP address as long as the default gateway replies, but if not, will automatically configure itself a new one.
• A discover message is sent every 3 minutes in an attempt to re-establish communications with the DHCP server.