Internet Core Protocols: The Definitive Guide

Address Mask Request and Address Mask Reply query messages

RFC 792 defined a variety of host-configuration messages, allowing diskless systems to obtain IP addresses and other data during the boot process by using ICMP messages. However, the primary ICMP query messages used for this—the Information Request and Information Reply query messages—have since been deprecated and are now obsolete. The Address Mask Request and Address Mask Reply query messages are also somewhat obsolete, although their usage has not been deprecated as of yet.

Essentially, the Address Mask Request and Address Mask Reply query messages allow a host to determine the subnet mask in use on the local network. Once a host has obtained an IP address (which could be obtained via Reverse ARP, BOOTP, or DHCP), they could then send an Address Mask Request message to the local broadcast address of 255.255.255.255. An Address Mask server on the network would then respond with an Address Mask Reply query message that is unicast back to the requesting device directly, providing the host with a subnet mask for the IP address being used.

RFC 1122 states that the Address Mask Request and Address Mask Reply query messages are entirely optional. Indeed, only a handful of systems use these messages today.

For more information on the Address Request and Address Reply query messages, refer to Address Mask Request and Address Mask Reply later in this chapter.