IP classes are used to use more efficiently IP addresses. You can hear of claasses from A to E, but the most common ones will be from A to C, since the other are usually reserved.
Depending on you network size, ammount of workstations, and ammount of newtworks, you should pick a class A,B or C ip class, each one will use a different (default) network mask, for example the default netowkr mask for a class C IP address, is 255.255.255.0. The you have subnetting that is divide a single network into different networks using logical addressing..but as Chris says, it could be a long topic depending on how much you want to know. A+, MCP, CCNA
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