5. What are private IP addresses?
The private IP address space is defined by RFC1918. In this RFC, it says that no public (take that as "no Internet") devices will use or recognize the following IP addresses:
- 10.0.0.0 to 10.255.255.255 (10/8 prefix)
- 172.16.0.0 to 172.31.255.255 (172.16/12 prefix)
- 192.168.0.0 to 192.168.255.255 (192.168/16 prefix)
Your IP address may be the same on your PC as someone else''s if you have a private IP address. These ranges of IP addresses are available for anyone to use on their own internal (private) network. There is no need to keep them unique. I can have IP address 192.168.1.1 on my home network and so can everyone else in the world! When I go to make a request to the Internet, however, that private IP address must be converted into a public IP address or else the Internet router I make the request to will just throw my request away (because I have a private IP address). Network Address Translation (NAT) is what performs this public-to-private translation (see RFC1631 and RFC2663 for more information on NAT).
Private IP addresses are there to reduce the need for more public IP addresses. An unintentional consequence is that they provide a tiny bit of security.
So, if I am trying to talk to your computer on the Internet and you tell me that your IP address is 192.168.3.3, I will tell you "No, I need your public IP address, not your private IP address."
About the author: David Davis (CCIE #9369, CWNA, MCSE, CISSP, Linux+, CEH) has been in the IT industry for 15 years. Currently, he manages a group of systems/network administrators for a privately owned retail company and authors IT-related material in his spare time. He has written more than 50 articles, eight practice tests and three video courses and has co-authored one book. His Web site is HappyRouter.com.