I don't understand what exactly you are after. Knowing a list of open ports will help you not at all in getting what you said you want and you can't "copy" this in any way.

If you want to guarantee simple IP connectivity the new host just has to be in the same subnet than the old one. That is: HOSTB has to share the IP subnet with HOSTA. Then it is copying the routing information of HOSTA (issue "netstat -rn" to get this information) and you should be done.

Further complications would arise if there are firewalls within your network: You will have to adapt their rules, but this can only be done on the firewall systems themselves, neither on HOSTA nor HOSTB.

If you need a certain protocol to work you will have to make sure all the prerequisites are met. For instance, if you want "ssh" to work (probably without further authentification) you will have to make sure the "sshd" configuration is in place ("/etc/sshd_config") and the keys are exchanged (as per default "~/.ssh/authorized_keys", "~/.ssh/known_hosts"). Still, you won't need any list of open ports for this.

Of course you could use "nmap" consecutively on all your hosts to find out open ports, but it would be probably easier to use "netstat" locally on each of them , as you claim to have HOSTA having connectivity to all of them (whatever this means - pingable? exchanged ssh-keys? something else?). Still, what will that information help you? Definitely not in building HOSTB, because there is nothing you could configure in HOSTB based on this information.

I hope this helps.

bakunin