Hello,
I have different vbox sessions. I run one application on one of them and it shouild be able to reach programms and files located on another session.
How can I do that ? Can I get a url that addresses each of my virtual machines ?

OS host linux X86_64

It depends on how you configured your virtual network(s) and whether
or not you have firewall restrictions on the guests.

How do you have your network(s) set up in VirtualBox?

I have not setup anything. There is a workgroup name, no domain set.

Just to be clear, when you mention vbox, are you referring to VirtualBox?

If so, you have the option to create virtual networks. As I recall, the
options are a bridged network to any one of your host's interfaces, a
host-only network and a NAT network. Check what type(s) you have configured.

Yes it is Virtual box. and it is defined as nat.

The next step would be to make sure each guest can connect to
the host. The easy way is to see if each guest gets an address
from dhcp. If so, your NAT connection is working properly.
If not, we'll have to figure out why.

Would you mind posting the IP addresses of each guest as well
as the IP address of the host on the NAT network?

each guest nat works. I have internet access on each guest and shared folders working.
But I need one application running on one guest to call another one running on another guest.
This is already working inside the same session, my program calls the other one asking him to look for an url. with localhost it works well inside my virtual session.
But I need this program to call another one outside the session, so I cannot use localhost to ask him to search. I need to identify the other virtual machine thanks to an url, but I do not know how to do this.

Okay, the next step is to make sure the guests can ping each other.
You might need to remove some firewall restrictions.

firewalls off

I'd suggest opening a terminal window on your host and pinging each
of the IP addresses. For example, if one guest uses 192.168.50.102,
type "ping -c1 192.168.50.102". Post the output for each of your guests.

Are you using DHCP in the guests? If you are then each will get an address in the 10.0.2.x range and AFAIK there is no way for them to learn each others addresses, that is VirtualBox does not run a dynamic DNS.