If you look under the Options menu item, you will see the selection 'Link' at the bottom. That will allow you to link emulators either on the same PC or across the network.
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Now, I'm certain you are asking about over the internet. All you need is to have that person's IP or host name at the time. But because of ISP's and such, you can't get it directly. The person who you want to connect to, if he is going to be the server for the network connection, would need to find some way to make it so no matter what happens, you will always be able to connect to him.
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If it is you who will be the server, then you also have to find some way to make others be able to connect to you.
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For both options, refer to DynDNS.com for an easy solution. It has a free option on it. It should allow the client to connect to the VBA-M emulator that will be the server, by giving that computer a host name, or a DNS.