An important function of your home network's router is protecting your network from unauthorized entry. Depending on circumstances, your router totally blocks some kinds of network traffic, while allowing others.
Your router can also allow data to flow selectively by using ports, which are like virtual gates inside your router. Ports allow network traffic to stream to one particular device on the network. They're specified by number (for example, normal web traffic uses port 80). When data arrives from the Internet addressed to a particular port, your router sends the data to the appropriate device on your network, based on the port number.
When you configure your router to work with your Slingbox, the setup process opens port 5201 on your router. All traffic to this port is directed to your Slingbox, which is designed to accept connections only from SlingPlayer. Your Slingbox uses network router ports to keep your network secure, while also giving you access to watch your TV via the Internet.
If you have more than one Slingbox on your network, each one receives its own port number, incremented by one: for example, port 5202, port 5203, and so on ... up to 5225.
Note: These port numbers assume that you are working with a Slingbox 350 or a SlingTV/Slingbox 500. The Slingbox M1 starts with port 5301, earlier Slingbox models start with port 5001 ... but other than that, the rest of the sequencing remains the same.
Only ports that are specifically configured to do so can admit data; by default, the rest are closed. Typically, router manufacturers reserve a small group of router ports for basic Internet functions such as web browsing and email.