This manual describes the situation when you have an instance or more instances connected to "external" typology. Every new instance connected to new "external2" typology has no network connection with old "external" typology by default. Following steps will guide you to new network creation in order to have "external" VMs and "external2" VMs connected.

If you don't have an instance connected to "external2" typology, please follow this guide: How to add floating IP to VM from external2 pool?

If you don't have an instance, please follow this guide: How to create new VM in OpenStack dashboard (Horizon)?

In this scenario, we use one instance connected to external network and the second instance connected to external2 network:

In order to connect those two networks, another network has to be created. 

In Project → Network → Networks choose "Create Network"

Provide any name for the new network, in this example, it will be called "private_LAN".

Provide "Subnet Name', "Network Address" and "Gateway IP". Network address has to be different than already used networks. We have 192.168.0.0/24 network used already in private_on_external and private_on_external2 networks so we use 192.168.8.0/24.

We need also provide allocation pools and DNS Name Servers.

The new network setup is compleated now. It will be visible in Project → Networks as below:

In Project → Compute → Instances there are two virtual machines on different external IPs

For both virtual machines choose "Attach Interface" in "Actions" menu

In "Networks" choose newly created "private_LAN"

And finish the setup by selecting "Attach Interface"

From now on both machines are connected through the new network. You can ping them by IPs from 192.168.8.X interfaces which can be checked by ifconfig command.