Private IPv4 addresses are special addresses set aside by the Internet Assigned Numbers Authority (IANA) for use within networks that will not be seen by or communicate directly with the Internet. These private addresses cannot be used on the Internet or used to communicate with the Internet (unless the address is NAT'd behind a firewall with a public address). All major ISP's filter out and delete packets using private IP addresses. Any organization that uses private IP addresses on computers that communicate with the Internet must use a device that performs Network Address Translation (NAT).
Anyone can use private addresses and they don't need to consult or ask permission from the Regional Internet Registries to use them. So long as the network using private IP addresses does not communicate directly with the Internet using those private addresses, there is no problem.
However, anyone can use private IP addresses and because IP addresses must be unique, networks using private addresses should not talk to each other or to the Internet unless their addresses do not overlap, or they are using NAT or NAT with overload (Port Address Translation or 'PAT').
There are several blocks of private addresses that were set aside by IANA specifically for this purpose.
| Private Class A range | 10.0.0.0 | - | 10.255.255.255 |
| Private Class B range | 172.16.0.0 | - | 172.31.255.255 |
| Private Class C range | 192.168.0.0 | - | 192.168.255.255 |
| Loopback Addresses | 127.0.0.0 | - | 127.255.255.255 |
The Private Class A range provides for up to 16,777,214 hosts on one network. In practice, most organizations that use the Class A range of addresses usually subnet this range of addresses into smaller sets of hosts called subnetworks (or just 'subnets' for short). Still, even with the subnetting, it makes for a conveniently large number of addresses.
The Private Class B range of IP addresses fall into 16 subnets. These were provided so that an organization could have up to 65,534 hosts on a network and have up to 16 networks.
The Private Class C range of IP addresses is designed to support 254 networks of up to 65,534 hosts.
The Loopback Addresses are covered in their own tutorial.