If you need to access your home network from the Internet, you need

1. an external IP address of your Internet connection
2. Port opened on the home router

I describe here, as an example, how to set this up. I limit myself to IPv4, because unfortunately I don't have an IPv6-capable connection.
Let's start with the external IP address problem. The Internet provider, Telekom, Unitymedia, 1&1 etc., provides us with one IP address for our router from the pool of his IP addresses. Since the number of addresses is limited with IPv4 and to ensure fair use, we are assigned a new IP address (usually daily). That means I have to write down this IP address every day. Since this is extremely impractical, some providers provide a so-called dynamic DNS service (often referred to as DynDNS). The idea behind this is to automatically notify the service of a change in IP, which it then manages as a static name.

NOTE - not every Internet connection, pure IPv6 connections that do not have a separate IPv4 address (keyword DS-Lite), can be accessed externally via IPv4.

But now finally we'll come to the head. After DynDNS made their service chargeable, I will now describe how I set up my new provider SPDNS. First you have to create an account, which only requires a working email address.
Once you have registered, you can manage up to 5 addresses free of charge. You start by deciding on a name and domain, in my case this is „onkelhartwig“ in the domain „myfirewall.org“. The Internet address is therefore onkelhartwig.myfirewall.org. During setup, the external IP from which you access the website is automatically suggested. That's about it as far as the DynDNS service is concerned.
If the IP address changes, it must also be updated in the DynDNS service. This can either be the router (not everyone offers the functionality), a PC or a Raspberry Pi. Here I describe the setup on the Fritz Box and on the Raspberry.

• Please activate the expert mode on the Fritz Box.
  • To do this, you have to click on the 3 dots in the Fritz Box interface at the top right and activate „extended view“.
• Then proceed as follows to make the settings for the DynDNS service
  • Internet → Releases → Tab: DynDNS
  • The following fields must be filled:
    • DynDNS Provider - Custom
    • Update URL - https://update.spdns.de/nic/update?hostname=<domain>&myip=<ipaddr> (just like that, without changing anything)
    • Domain name - onkelhartwig.myfirewall.org (of course the name has to be changed here
    • Username - <The SPDNS username>
    • Password - <The chosen password>
• Don't forget to click the „Apply“ button to save the data in the Fritz Box
• Another up-to-date guide can be found in the FAQs from SPDNS

From now on, your home network can be reached from the Internet via <selected hostname>.<selected domain> (in my case onkelhartwig.myfirewall.org).

Depending on the router, setting up port forwarding is easy, possible or impossible

• for rental/loan devices, the website of the provider
• the manufacturer's website
• Google

I will describe this briefly using the Fritz Box.

• Please activate the expert mode on the Fritz Box (if not already done).
  • To do this, you have to click on the 3 dots in the Fritz Box interface at the top right and activate „extended view“.
• Then proceed as follows to make the settings for the DynDNS service
  • Internet → Releases → Tab: Port releases
• To create a port release you must first click on „Add device for releases“ or, if a release already exists, on the pencil next to the name of the device
• Now you have to click on „New Release“ (at the bottom right).
• When filling out the fields, make sure that there is a distinction between „MyFritz! release“ and „port release“. I always choose port forwarding because the MyFritz! service is not my thing
• For external ssh access to the Raspberry (as an example), the following information must be provided:
  • Application - Other application
  • Designation - external SSH access on port 4711
  • Protocol - TCP
  • Port on device - 22 to 22
  • External port desired: 4711
• Click „OK“.
• Click on „Apply“, if you forget the last two points, you can do everything again

Now I can access my Raspberry Pi from the Internet with „ssh -p 4711 pi@onkelhartwig.myfirewall.org“. je