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Private IP address are addresses in the range of 10.xxx.xxx.xxx, 172.16.xxx.xxx - 172.31.xxx.xxx, 192.168.xxx.xxx, where the x in each octet can be a number ranging from 1 to 255. Addresses in these range are not routable on the Internet, thus it is known as private IPs.

IP addresses whether private or public are also masked using subnet masks, typically Class A, Class B and Class C.

Class A subnet are defined 255.0.0.0. So for a IP address of 10.0.0.0 with a mask of 255.0.0.0, the address range would be from 10.0.0.1 - 10.255.255.255 with only the first octet (the digits separated by the dot) staying consistent.

Class B subnet are defined as 255.255.0.0. So it may range from 172.16.0.0 - 172.16.255.255 for an address of 172.16.0.0 with mask of 255.255.0.0. The first two octets are consistent.

Class C subnet are defined as 255.255.255.0 and it is commonly used in smaller environment with less than 254 connected devices. Thus 192.168.0.0 address with a mask of 255.255.255.0 ranges from 192.168.0.1 - 192.168.0.255. It is also possible to use 10.0.0.0 with a Class C subnet mask and it will have an address range of 10.0.0.1 - 10.0.0.255 with the three first octets being consistent.

Besides private IP addresses, there are also other reserved addresses not in use on the Internet. One is the loopback address which is 127.0.0.1, this just means pointing back to self.

Also within a subnet of IP addresses, the last IP is always the broadcast address thus it is not in use. Thus in the address range 192.168.0.1 - 192.168.0.255, the last address 192.168.0.255 is the broadcast address and is not useable by anyone. Packets sent to this address will be broadcast to every device within the subnet.

In Windows clients, Microsoft also automatically allocated IP addresses in the range of 169.254.0.0 - 169.254.255.255 to nodes that are unable to obtain IP address from a DHCP server.