Basically, I am using virtualbox on my other dedicated server but there I have no problems at all -> because one of the providers allows me to use DHCP, so I bridge my VBox interfaces and if I have several of them, I get different IP's for different MAC addresses.
Unfortunately now I have to cope with different provider... So, to sum up:
First ip address I have (for the host):
Second IP I got (which I need assign for a guest OS running on VirtualBox):
Mask: 255.255.255.255 (mask: /32!)
Gateways are the same. As I was instructed by r0b0, I changed config on the guest OS (inside VM) and set VM into bridging.
iface eth0 inet static
post-up ip route add XX.XXX.Z4.40/255.255.255.252 dev eth0; route add default gw XX.XXX.Z4.41 eth0
After doing that, if I ping from the host os, I get:
From XX.XXX.Z4.41: icmp_seq=1 Redirect Host(New nexthop: XX.XXX.Z4.42)
How to finally get it done? I'm fed up!