From RFC:950

"A gateway receiving an address mask request should return it with the address mask field set to the 32-bit mask of the bits identifying the subnet and network, for the subnet on which the request was received.

If the requesting host does not know its own IP address, it may leave the source field zero; the reply should then be broadcast. However, this approach should be avoided if at all possible, since it increases the superfluous broadcast load on the network. Even when the replies are broadcast, since there is only one possible address mask for a subnet, there is no need to match requests with replies. The "Identifier" and "Sequence Number" fields can be ignored."