2.3. Finding the Address Mask

How can a host determine what address mask is in use on a subnet to which it is connected? The problem is analogous to several other "bootstrapping" problems for Internet hosts: how a host determines its own address, and how it locates a gateway on its local network. In all three cases, there are two basic solutions: "hardwired" information, and broadcast-based protocols.

Hardwired information is that available to a host in isolation from a network. It may be compiled-in, or (preferably) stored in a disk file. However, for the increasingly common case of a diskless workstation that is bootloaded over a LAN, neither hardwired solution is satisfactory.

Instead, since most LAN technology supports broadcasting, a better method is for the newly-booted host to broadcast a request for the necessary information. For example, for the purpose of determining its Internet address, a host may use the "Reverse Address Resolution Protocol" (RARP) [4].

However, since a newly-booted host usually needs to gather several facts (e.g., its IP address, the hardware address of a gateway, the IP address of a domain name server, the subnet address mask), it would be better to acquire all this information in one request if possible, rather than doing numerous broadcasts on the network. The mechanisms designed to boot diskless workstations can also load per-host specific configuration files that contain the required information (e.g., see RFC-951 [8]). It is possible, and desirable, to obtain all the facts necessary to operate a host from a boot server using only one broadcast message.

In the case where it is necessary for a host to find the address mask as a separate operation the following mechanism is provided:

To provide the address mask information the ICMP protocol [5] is extended by adding a new pair of ICMP message types, "Address Mask Request" and "Address Mask Reply", analogous to the "Information Request" and "Information Reply" ICMP messages. These are described in detail in Appendix I.

The intended use of these new ICMP messages is that a host, when booting, broadcast an "Address Mask Request" message. A gateway (or a host acting in lieu of a gateway) that receives this message responds with an "Address Mask Reply". If there is no indication in the request which host sent it (i.e., the IP Source Address is zero), the reply is broadcast as well. The requesting host will hear the response, and from it determine the address mask.

Since there is only one possible value that can be sent in an "Address Mask Reply" on any given LAN, there is no need for the requesting host to match the responses it hears against the request it sent; similarly, there is no problem if more than one gateway responds. We assume that hosts reboot infrequently, so the broadcast load on a network from use of this protocol should be small.

One potential problem is what a host should do if it can not find out the address mask, even after a reasonable number of tries. Three interpretations can be placed on the situation:

1. The local net exists in (permanent) isolation from all other nets.

2. Subnets are not in use, and no host can supply the address mask.

3. All gateways on the local net are (temporarily) down.

Finally, note that no host is required to use this ICMP protocol to discover the address mask; it is perfectly reasonable for a host with non-volatile storage to use stored information (including a configuration file from a boot server).