What are private IP addresses, and what are the reserved ranges?

InterNIC has reserved certain IP addresses as private addresses for use with internal web sites or intranets. These addresses are not routable on the public Internet, but are meant for devices that reside behind a router or other Network Address Translation (NAT) device or proxy server. Private IP addresses are used either to hide systems from the public Internet or to provide an additional range of addresses to organizations that do not have sufficient public IP addresses to distribute on their network. Organizations can use these numbers to assign internal IP addresses without having to worry about an IP address conflict or having to obtain a new block of IP addresses.

If you connect to the Internet as a home user with a residential router, you will typically benefit from this arrangement. Although you may be paying for only one IP address through your Internet service provider (ISP), you can have unlimited devices connected to the Internet. Using a private IP address will make your computer invisible to certain types of network attacks; however, you will not be able to easily establish your computer as a server.

At Indiana University, internal addresses are used in two circumstances:

1. For users who have not registered their computers with the DHCP service, the network assigns internal addresses so they have enough connectivity to reach the DHCP registration web page.
   
   
2. For wireless users, the wireless network assigns internal addresses so they have enough connectivity to establish a VPN connection.

The IP addresses that can be used for internal networks are listed below:

For IPv4:

• 10.0.0.0 - 10.255.255.255
  
  
• 172.16.0.0 - 172.31.255.255
  
  
• 192.168.0.0 - 192.168.255.255

For IPv6:

• Site local addresses are similar to RF1918 addresses (10.0.0.x, 192.168.2.x).
  
  
• Site local prefixes begin with fecx, fedx, feex, and fefx (fecx is most commonly used).

Private Intranet addresses cannot be sent over the Internet. If an Internet router receives a packet with either a source or destination address that is reserved, it will drop the packet.

Also see:

• How can I determine who is responsible for a particular IP address or domain?