I do realize that most LANs sit behind a device running NAT, which translates the users private IP address to a public IP address. My question is still if you are trying to connect to a private device, how does the internet know which private IP you are connecting to, since many LANs have the possibility of using that IP address? Is it that you can only connect using the public IP address?
Routing – How Can Multiple Devices Share an IP Across the Internet?
iplannat;routingwan
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You should search for the OSI model:
A full explanation of the OSI model is too broad to give here, but applications like browsers and web servers use the transport layer to communicate with each other, not the network layer where IP addresses are.
What you are missing is that HTTP uses TCP as the transport protocol. By default HTTP servers listen on TCP port 80. A host using a browser will use a random, ephemeral, TCP port as its source port. NAT will do something similar for each host for which it translates the IP address, and it keeps track of which port is assigned to which host in a traffic flow. It ends up being a combination of layer-3 address (e.g. IP address), transport protocol (e.g. TCP), and layer-4 address (e.g. TCP port) for what gets used as a unique identifier.
There is no real difference in IPv4 or IPv6 network diagrams. The network devices are the same things (routers, switches, firewalls, etc.), and the diagrams look the same, other than the addressing.
Routers route between networks, and IPv6 still uses networks. Firewalls are still used to protect networks. Switches are still used to connect hosts.
Since IPv6 has plenty of address space, there is no need to NAT. NAT breaks the original IP premise of end-to-end connectivity, where every device has a unique IP address. IPv6 doesn't have a NAT standard, and NAT breaks things in IPv6. NAT was developed as a short-term solution to the IPv4 address shortage, and it is expected to go away once IPv6 is ubiquitous.
Best Answer
In order for some device on the internet to connect inbound to a device on your private network you must use a NAT configuration. On residential equipment like a cable modem/router this is often referred to as port-forwarding. Think of it this way. If you are using a single public IP address and sharing it between several private addresses, you must tell the cable modem/router that if I receive a connection on the public IP address on TCP port 80, then that should go to the private address of x.x.x.x on port 80 (or any port you want really). This gives the modem/router enough information to successfully deliver the packets to the internal device from a device out on the internet.