But your browser and mine both resolve forum.feed-the-beast.com to the same address. When I load your world, how does my game know that when it sees 100 in your file, it means 1 in my game's numbering?...it doesn't matter that your system thinks "Minecraft:Stone" == ID 1 while mine thinks "Minecraft:Stone" == ID 100.
Think of the Items as being assigned "addresses" like your computer is assigned by your router's DHCP server, and when you want to access an item your client looks up the name and replaces it with the ID like your browser finds a website's IP by looking up the name using DNS.
Feel free to treat that question as rhetorical. This is why I asked for links, because I didn't want to derail this thread with technical discussions.