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A router MUST implement support for receiving ICMP Address Mask Request messages and responding with ICMP Address Mask Reply messages. These messages are defined in [INTERNET:2].

A router SHOULD have a configuration option for each logical interface specifying whether the router is allowed to answer Address Mask Requests for that interface; this option MUST default to allowing responses. A router MUST NOT respond to an Address Mask Request before the router knows the correct address mask.

A router MUST NOT respond to an Address Mask Request that has a source address of and which arrives on a physical interface that has associated with it multiple logical interfaces and the address masks for those interfaces are not all the same.

A router SHOULD examine all ICMP Address Mask Replies that it receives to determine whether the information it contains matches the router's knowledge of the address mask. If the ICMP Address Mask Reply appears to be in error, the router SHOULD log the address mask and the sender's IP address. A router MUST NOT use the contents of an ICMP Address Mask Reply to determine the correct address mask.

Because hosts may not be able to learn the address mask if a router is down when the host boots up, a router MAY broadcast a gratuitous ICMP Address Mask Reply on each of its logical interfaces after it has configured its own address masks. However, this feature can be dangerous in environments that use variable length address masks. Therefore, if this feature is implemented, gratuitous Address Mask Replies MUST NOT be broadcast over any logical interface(s) which either:

  • Are not configured to send gratuitous Address Mask Replies. Each logical interface MUST have a configuration parameter controlling this, and that parameter MUST default to not sending the gratuitous Address Mask Replies.

  • Share subsuming (but not identical) network prefixes and physical interface.

The { <Network-prefix>, -1 } form of the IP broadcast address MUST be used for broadcast Address Mask Replies.


The ability to disable sending Address Mask Replies by routers is required at a few sites that intentionally lie to their hosts about the address mask. The need for this is expected to go away as more and more hosts become compliant with the Host Requirements standards.

The reason for both the second bullet above and the requirement about which IP broadcast address to use is to prevent problems when multiple IP network prefixes are in use on the same physical network.

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