grammar - Countable Attributive nouns in plural context

There are a lot of topics about this question. All of them explain the form (plural, singular) of the attributive noun coupled with a main noun in the singular form, for example:

• ladies room


• steel bridge

To show the plural form, we should change the form of the main noun (right?), that is:

• ladies rooms (many rooms for ladies)


• steel bridges (many bridges)

I've discovered a pattern: in most cases, such nouns are compounded with an uncountable attributive noun. But when we try to modify the main noun with an uncountable noun, and want to show plural form, it leads to ambiguity. For instance:

subnet identifiers

In this case, subnet has several identifiers. But if I want to show that there is more than one subnet, and that each (one) of them has several identifiers, is it correct to write:

subnets identifiers

Or in such situations, should I prepositional expression, such as:

identifiers of subnets