Net.names are one of the LNH's most enduring jokes/worldbuilding elements. Simply put, the names of many cities, states, countries, and other locations in the Looniverse are Internet-based puns. The most obvious of these is Net.ropolis, but other commonly used ones include Sig.ago, Net.York, Alt.stralia, Net.Zealand, and so on, as well as more involved puns like the Usenetted Kingdom.
These names are not universally used. In particular, Net.Trenchcoat Brigade stories generally avoid them, as part of their more "gritty and realistic" tone (the exception being Net.ropolis, because it doesn't really have another name). Non-English-speaking countries are also less likely to be net.named, especially those outside of Europe (or Eu.rec); you'll often see Japan rather than Japa.net.
In LNH20, these names were inspired in-world by The Network, whose popularity in the mid-20th century lead to many cities giving themselves official or semi-official nicknames. Many of these were abandoned later on, or are used intermittently by locals and/or tourists.
The Looniversal Gazeteer was a guide to and list of Net.names, tho many of those listed have never actually been used in a story.