If the very first number of the 32 binary digits is set to 0, this is a Class A address. The common subnet mask is 255.0.0.0. The subnet mask is a shorthand way of telling both the network admin, and the underlying innards of TCP/IP, how much of the address is used for the network (or subnet) number, and how much is used for the host number. The subnet number 255 hides the first 8 bits of the address so it used for the network number. This becomes the unique network number that can range between 001.n.n.n and 126.n.n.n. If you look at the binary math, you see it is possible to create the number 127, however that number as a reserved meaning in TCP/IP. The last 24 bits are available for separate clients. While Class A networks have the fewest network numbers, they have the possibility of supporting the most client devices.
Each network address class has a range of private network addresses or IDs, reserved for use on networks without a direct connection to the Internet. In order to receive a public IP address, an individual or organization must acquire it from their ISP or from the administrators of the Internet. What if an organization isnt connected to the Internet, or only allows a limited number of users to connect to it? They dont need a public network address, if all they want to do is allow their internal users to communicate with each other. So, TCP/IP reserves a range within each address class for this purpose.
In Class A, the reserved private network range is 10.0.0.0 10.255.255.255.
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