How Do Multiple Computers Access the Internet Through a Single Connection?
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Background Information
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To fully understand how several computers can access the Internet through one connection, an understanding of how a network operates must come first. All the computers connected to each other in one area (within a home or office building) comprise a local area network, or LAN. The LAN computers connect to a device that allows those computers to communicate with each other-- usually a switch. The switch also connects to a device called a router that connects the LAN with another network that is not local. With most home and small business applications, the router and switch are in the same device.
Every device connected to the switch has an IP address that identifies its location. The router has two IP addresses, one for the LAN and one for the other network it connects to. That other network router connects to other networks, which connect to others, which creates the web of networks and computers we call the Internet.
Creation of a Packet
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When a computer wants to send data across the network it connects to, it creates a packet. This packet contains information about where the data is going and where it is coming from, much like the address and return address on a piece of mail. Since the packet is going to access the Internet, a piece of data called the header attaches to the packet that tells the switch to send it to the router, or gateway. The router reads that the packet is Internet-bound and puts it on the line to the other network, where the other routers in the Web send the packet to the right place. At the same time, the router makes a note of the sending local IP address and puts that information in the NAT table.
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NAT
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The Network Address Translation, or NAT, table is a list of computers on the LAN, and where they are. As the packet is routed to the other network, the router adds a header that corresponds to the PC entry in the NAT table. To devices outside the LAN, it appears as if all of the packets were created by the router. When the receiving computer sends a reply, that header is includes in the address. When the return packet reaches the router, the header information is read, and the correct PC receives the return packet. The more devices on the LAN, the bigger the NAT table.
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