Add some form of method that does that, because forge tracks the registered block IDs, so will know where there is a break. If waits until after the mod has declared all it's blocks/items, and then gives it an ID range that fits, for better use of the space there, because with lots of mods installed it becomes difficult to track.

Well, i did not know of either of those, thanks (What does 1.7 use instead then?)

(What does 1.7 use instead then?)

Internally it still uses IDs. But they are dynamic, that means in world A stone can be Id 1 and in world B it could be Id 334 or whatever. In the code you work with the actual Block object or with identifiers like "minecraft:stone".

And there would soon be no IDs at all (from 1.8 onwards)...