The main router on most home networks serves as an Internet gateway that enables multiple computers to share a single Internet connection and protects the computers with a firewall. To do this, the router assigns each computer a unique private IP address and monitors all traffic to and from the Internet. This enables you to view the computers that are connected to the router and the Internet addresses of the websites on the other side of the connection.
Sharing the Internet Connection
All Internet communication must originate from a public IP address. In most home networks, a router serves as the Internet gateway and assumes the public IP address for the network. When a computer connects to the network, it assigns it a unique private IP address that's only valid within the context of the local network. When a computer sends data to the Internet, the router replaces the private IP address with the public address and assigns it a port number. When data comes back from the Internet, the router substitutes the private address and routes the traffic. This process shares a single public IP address and has the additional benefit of creating a firewall.
Displaying Connected Computers
There are a fixed number of valid private IP addresses. Most routers keep track of the device that's using each address, even when the router didn't assign it to the device. When you log in to your router and view the LAN status, the router displays each computer on the LAN, the IP address the computer is using, the computer's MAC address and the computer's name. Some routers enable you to click on the MAC address and obtain information about the manufacturer of the main board in the computer.
Examining Internet Traffic
In addition to monitoring which computers are connected to the network, you can also monitor each computer's Internet traffic. Select "Internet Sessions" or a similar option to display a list of every connected computer's active Internet sessions. You can view the local IP address and port assigned by the router, the Internet IP address to which it's connected and the type of Internet traffic, either UDP or TCP. TCP is typically used for Web browsing, while UDP is most often used when streaming music or videos. Some routers perform a reverse IP look up to display the domain name of the IP address to which a computer is connected.
Limiting Internet Access
You're not limited to simply monitoring computers connected to your router; most routers also enable you to restrict what a connected computer can do. For example, you might assign a rule to one or more computers that only allows them to connect to the Internet between 6 a.m. and 11 p.m. Computers without a rule aren't limited. You might also be able to restrict the websites that a computer can visit, either by domain name, IP address or by installing a filter that limits access by the type of content, such as pornography or violence.
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