These IP network blocks were set aside for private use (for testing networks) and don’t appear on any (properly configured) public network. This makes them ideal for use on home networks or internal corporate LANs ^*.
Class A: 10.0.0.0 (Up to 16,581,373 individual IP addresses)
Class B: 172.16.0.0 (Up to 65,023 individual IP addresses)
Class C: 192.168.0.0 through 192.168.255.0 (Up to 255 individual addresses.)
The Class C block is actually a set of 255 Class C networks. 192.168.0.0, 192.168.1.0, 192.168.2.0 and so on. Only the last octet is used for an individual host (computer, router, etc), the other three are used to specify the network address.
All of these blocks can be subdivided (or “subnetted”) by using a subnet mask. The “default” subnet mask includes only the network portion of the address. So for 10.0.0.0,the default mask would be 255.0.0.0. But you could use something like 255.255.0.0 to break it up into 255 small networks, each of which would have, at most, 65,023 (that’s 255 x 255 – 2) hosts on it.
A curious reader may wonder, “Why is it 255 x 255 – 2? Why not the whole 255 x 255?”
That’s because one address on each network (the one where the host bits are set to 0, e.g. 10.10.0.0) is reserved as the “network address” and another one (where host bits are set to all 1s, e.g. 10.10.255.255) is reserved as the “broadcast” address.
^*You should almost always (some corporate settings are excepted) set up your private network to use one of the network blocks discussed here. Setting up your private LAN so that it uses other people’s IP addresses would have the effect of hiding the other poeple’s web sites and such. (For example, if your home network used the 72.232.101.0 network, you wouldn’t be able to reach WordPress.com, and what fun would that be?) Setting up a public network using someone else’s IP block would likely get you into trouble with whoever was providing your Internet connectivity. Don’t do it.