This description will give you further information about public IP addresses which are a part of the STACKIT External Networks (Basic Concepts -ExternalNetwork-).

The different kinds of public IP addresses

In order to reach STACKIT VM's (Basic Concepts Compute Engine) from an external location (e.g. the Internet), or if you want to use internet services, you need a public IP address. There are two ways off using public IP addresses at STACKIT which are described below.

Floating IP addresses

Floating IP addresses are public IPv4 addresses, these addresses are needed so you can reach your STACKIT server (VM) from the internet.

Technically this is realised via Network Address Translation (NAT) between the internet and the VM with the bound Floating IP. But on the VM itself there will be no public IP address configured (e.g. as an interface), instead the private (local) IP address of the VM will be exchanged with the public one when accessing the internet. The same thing happens, in the opposite direction, when you or someone else tries to connect to the VM via the Floating IP over the internet. In this case the public IP will be replaced with the private (local) IP address of the VM.

Due to the implementation of the Floating IP as a NAT-Solution the address translation happens on the network layer, before the data packets reach the VM. Therefore the Floating IP is not visible on the VM itself (e.g. as an interface).

Router IP

Also systems without a Floating IP from STACKIT can access the internet - but not the other way around. For this process you don't need a separate Floating IP, this process uses a so called "Router IP". In this case all requests from the VMs, that are connected to the router, will use the public IP address of the router via Source Network Address Translation (SNAT). This is an oneway function, because different source systems (STACKIT VMs) access the internet using the same public IP address.

Prerequisite for the Router-IP is, that one of the VMs, with a connection to the router, has a Floating IP bound.