Connected: An Internet Encyclopedia Address Mask Request/Reply: RFC-950

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A host MUST support the first, and MAY implement all three, of the following methods for determining the address mask(s) corresponding to its IP address(es):

  1. static configuration information;

  2. obtaining the address mask(s) dynamically as a side- effect of the system initialization process (see [INTRO:1]); and

  3. sending ICMP Address Mask Request(s) and receiving ICMP Address Mask Reply(s).

The choice of method to be used in a particular host MUST be configurable.

When method (3), the use of Address Mask messages, is enabled, then:

  1. When it initializes, the host MUST broadcast an Address Mask Request message on the connected network corresponding to the IP address. It MUST retransmit this message a small number of times if it does not receive an immediate Address Mask Reply.

  2. Until it has received an Address Mask Reply, the host SHOULD assume a mask appropriate for the address class of the IP address, i.e., assume that the connected network is not subnetted.

  3. The first Address Mask Reply message received MUST be used to set the address mask corresponding to the particular local IP address. This is true even if the first Address Mask Reply message is "unsolicited", in which case it will have been broadcast and may arrive after the host has ceased to retransmit Address Mask Requests. Once the mask has been set by an Address Mask Reply, later Address Mask Reply messages MUST be (silently) ignored.

Conversely, if Address Mask messages are disabled, then no ICMP Address Mask Requests will be sent, and any ICMP Address Mask Replies received for that local IP address MUST be (silently) ignored.

A host SHOULD make some reasonableness check on any address mask it installs; see IMPLEMENTATION section below.

A system MUST NOT send an Address Mask Reply unless it is an authoritative agent for address masks. An authoritative agent may be a host or a gateway, but it MUST be explicitly configured as a address mask agent. Receiving an address mask via an Address Mask Reply does not give the receiver authority and MUST NOT be used as the basis for issuing Address Mask Replies.

With a statically configured address mask, there SHOULD be an additional configuration flag that determines whether the host is to act as an authoritative agent for this mask, i.e., whether it will answer Address Mask Request messages using this mask.

If it is configured as an agent, the host MUST broadcast an Address Mask Reply for the mask on the appropriate interface when it initializes.

See "System Initialization" in [INTRO:1] for more information about the use of Address Mask Request/Reply messages.


Hosts that casually send Address Mask Replies with invalid address masks have often been a serious nuisance. To prevent this, Address Mask Replies ought to be sent only by authoritative agents that have been selected by explicit administrative action.

When an authoritative agent receives an Address Mask Request message, it will send a unicast Address Mask Reply to the source IP address. If the network part of this address is zero (see (a) and (b) in, the Reply will be broadcast.

Getting no reply to its Address Mask Request messages, a host will assume there is no agent and use an unsubnetted mask, but the agent may be only temporarily unreachable. An agent will broadcast an unsolicited Address Mask Reply whenever it initializes, in order to update the masks of all hosts that have initialized in the meantime.


The following reasonableness check on an address mask is suggested: the mask is not all 1 bits, and it is either zero or else the 8 highest-order bits are on.

Next: 3.2.3 Internet Group Management Protocol IGMP

Connected: An Internet Encyclopedia Address Mask Request/Reply: RFC-950