How can URLs have a dot . at the end, e.g. www.bla.de.?

From here

It's a little-known fact, but fully-qualified (unambiguous) DNS domain names have a dot at the end. People running DNS servers usually know this (if you miss the trailing dots out, your DNS configuration is unlikely to work) but the general public usually doesn't. A domain name that doesn't have a dot at the end is not fully-qualified and is potentially ambiguous. This was documented in the DNS specification, RFC 1034, way back in 1987:

Since a complete domain name ends with the root label, this leads to a printed form which ends in a dot. We use this property to distinguish between:

• a character string which represents a complete domain name (often called "absolute"). For example, "poneria.ISI.EDU."

• a character string that represents the starting labels of a domain name which is incomplete, and should be completed by local software using knowledge of the local domain (often called "relative"). For example, "poneria" used in the ISI.EDU domain.

The source of this content continues to explain this in more detail.

Hostnames without a trailing dot are potentially ambiguous. A trailing dot means that the hostname is fully qualified and may not be relative to the local search domain.

Imagine you are a student of the (fictive) Example University which has the second-level domain example.edu. Inside the university's campus network you can omit the .example.edu suffix for ease and lazyness. So if you want to surf to www.example.edu and are on-campus, you just have to surf to www and it works.

Now imagine the university has an institute for artificial intelligence ("AI") and their website is hosted on www.ai.example.edu. If you're on-campus, surfing to www.ai will suffice since you can omit the .example.edu suffix.

So far, so good. But now you want to surf to the "Offshore Information Services" company, which runs Anguilla's top-level domain .ai. Their website is at http://www.ai/ — but if you enter that URL into your web browser while being on campus, you'll land on the website of the institute for artificial intelligence instead.

So you need to tell the web browser that you really want to go to the website with the top-level domain .ai and not to the one with your university's subdomain .ai.example.edu. That's where the dot is important, because http://www.ai./ will always work independent of the local DNS search domain.

Analogous to the dots between the different domain levels, the last dot represents the root of the DNS, i.e. it states that the word before that dot is a top-level domain and no domain relative to the search domain.