A subnet mask tells a computer and/or a router what portion of an IP address is a host and what portion is a network. Take, for example Google's IP address: 72.14.207.99
You should probably already know what *your* IP is: 68.87.60.XXX --- insert your Flakey ComCast IP number here.
Let's assume that your router address from ComCast is: 68.87.60.1
Now let's use a standard class C subnet mask: 255.255.255.0
Using this information, your computer "knows" that your network is 68.87.60.0
So if you want to send a message to 68.87.60.77 it will check that address with its subnet mask first to see if it is in its network.
How does it check? It "ANDs" the subnet mask with 68.87.60.99 and it does this using **binary** operations and numbers.
01000100.01010111.00111100.01001101 = 68.87.60.77
11111111.11111111.11111111.00000000 = 255.255.255.000
01000100.01010111.00111100.00000000 = 68.87.60.000 = Bingo, I'm on the same network!
Now it just sends the message to your nosey next door neighbor with ComCast with no fussing with the router. Notice that the zeros are the part that actually "mask" when you AND them to something else. The ones just let the original number pass through when they are ANDed.
Now, you want to search for widgets on Google using one of its known IPs: 72.14.207.99:
01001000.00001110.11001111.11001111 = 72.14.207.99 = The Almighty Google
11111111.11111111.11111111.00000000 = 255.255.255.000
01001000.00001110.11001111.00000000 /= 01000100.01010111.00111100.00000000 = Oh no! Gimme router!
So now, if the router is configured correctly on your computer, your message to Google gets sent to 68.87.60.1 and then the router figures out which network interface Google is on so you can get your shiny new widget.
That's as far as I dare take it before the weekend! Suck this up and try to squeeze it into your cerebrum.
