A very common trend in new housing in Japan (particularly in Tokyo) is the availability of “free internet” (listed as インタネット無料). Do not be fooled into buying an apartment for this benefit. These schemes are profit centers for apartment management companies and building owners. Unlike most countries, internet service is relatively cheap in Japan, so offering one shared connection for the whole building for “free” is not a large cost for building owners and management companies. But it is a huge sacrifice for tenants. You're almost always sharing an internet connection with your neighbors. In this scenario, you are at the mercy of your neighbors. It is the “most shared” connection you can get. Sometimes the shared connection is split fiber optic, but more often than not, it is Ethernet or VDSL to your apartment. Especially in the latter case, you are at the mercy of other tenants.

When you're looking for housing, always confirm if you can get a separate internet connection, and if you can, whether or not construction is required or necessary. In most cases, you'll want to get a separate internet contract with NTT FLETS (the de facto national fiber optic network), but you might also want to get a contract with NURO (Sony), GameWith Pro (dedicated fiber for gamers), or GLBB F::Xpress (dedicated fiber for internet otaku). All of these services will be significantly better than free internet for daily internet use.

“Free internet” included in your building contract will often be slow during peak times (e.g., weekends and evenings), and may have other issues like packet loss that make online gaming difficult, streaming difficult, or general web browsing difficult.

Consider yourself warned.