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Fellas, i'm a newbie learner about the whole Networking thing and im confused about how ip-address assignments work and communication occur. If someone can please explain this to me, i'd be very much thankful. So this is my question;

Lets say Company A uses private Class B address inside their network. They assign i.e 172.16.31.0/24 subnet for their servers in dmz and NAT will take place at their Edge device (firewall or router) and translate this address to a public routable address on the internet. Company B also uses the same class B subnet for their servers. Now these two decide to merge. How will the communication occur if its the same Ip address on both sides.

I really hope i asked the question as simply as i could and if there is something i'm missing for me to understand the big picture, please pitch in. And what kind of WAN circuit between sites would they benefit from ?

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If they are communicating over the internet (by email for example, if on-premises), the target IP address is always their public IP address. What happens behind the NATing router is invisible to the other side. Nothing needs to be changed, everything keeps working as usual.

If they want to merge or combine their network via a VPN one should change their subnet. Otherwise, computers from company 'A' will never send their packets for computers in company 'B' to the gateway – and further through the vpn tunnel – because it looks like that they are on the same network.

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  • Worth mentioning that RFC 4193 was introduced to avoid this kind of problems.
    – kasperd
    Apr 6, 2015 at 13:03

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