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private network is a network that uses private IP address space(What is it?).These addresses are commonly used for home, office, and enterprise local area networks (LANs),

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superuser.com perhaps? –  andol Jul 23 '10 at 7:21
Yup, voting to move. From the faq: "Server Fault is for system administrators and IT professionals, people who manage or maintain computers in a professional capacity." –  ThatGraemeGuy Jul 23 '10 at 7:43
Voting to move. It should be a super user problem. –  xiao Jul 23 '10 at 8:27
Private IP addresses are defined in RFC 1918 and 4193. –  dbasnett Jul 26 '10 at 11:02
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migrated from serverfault.com Jul 24 '10 at 5:18

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3 Answers

IP addresses by themselves are just unique identifiers (hence "address") used to identify each computer or device on a network. They specifically apply to networks using the IP protocol, which is really all most people will encounter. There are 2 IP address protocols in use: IPv4, and IPv6. IPv4 is vastly more common today, but networks are slowly starting to migrate.

The basic premise of IP addressing is that each computer connected to a network must have a unique address. In IPv4, all addresses are 32 bits long, broken into 8-bit chunks separated by periods (i.e. - Back in the day, there were enough addresses that everyone on the Internet could have their own unique address, and there were plenty to go around. Then the Internet got full. Because of the "shortage" of IP addresses (there are 232 possible IPv4 addresses), the idea of "private addresses" were introduced. These are the following blocks:

The /8, /12, and /16 refer to the subnet mask, which is used to determine how large the network can be before it must be split up. The subnet masks above are 8, 12, and 16 bits, yielding maximum network capacities of 224, 212 and 28 devices, respectively. Note that for the 2nd and third groups, there are multiple networks available within this range (i.e. and must be on separate networks, but and don't have to be).

(Note that IPv6 by contrast uses 128-bit addressing, which leaves a huge address space available so that we might once again all have unique public IP addresses.)

In order to make these "private" networks useful, many networks have employed Network Address Protocol (NAT), or as it is more commonly seen Port Address Translation (PAT) (sometimes also referred to as "NAT Overload"). Most devices which claim to support NAT are really using PAT. The idea behind PAT is that the router can mask away a large number of private IP addresses to a single public IP. It does so by mapping each internal/private IP address to the public IP address and a specific port (as usually defined in the TCP or UDP protocols). This way, you can have a network containing multiple devices (e.g.,,, etc), and they will all be represented by the single router with a public IP address (e.g.

The end result of this is that many people can re-use the same private IP address ranges without conflict on the globally connected Internet. For example, almost all home routers use or By employing PAT, we are all able to use these IP ranges without having conflicts when our computers actually talk over the Internet. The computers at each end are talking only to the router with the public IP, which then keeps track of where to forward the traffic on the private network based on the incoming port from a higher level protocol (i.e. TCP or UDP).

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Yeah, this question might belong somewhere else, but...

You already know that Wikipedia is your friend, because the line in your question "These addresses are commonly used..." comes from the article.

The three private IP ranges:

Are private because everyone who configures a router agrees not to route them. Even if you configure your own router to pass them out of your network unchanged, the next guy's network is either going to choke on them (if they conflict) or drop them.

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IP address is a unique identifier for a machine on a network. A network is a group of machines. To change of network, you must cross a router. The IP addresses are public somebody on Internet can join the network from an other site. Private IP are use in local and don't cross the routers.

To access to Internet, a private IP must be transform to a public IP : it is NAT (Network Address Translation). When the information come back from Internet, the router change the IP from public to private.

IANA define which IP is public or private : http://www.iana.org/abuse/faq.html

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what is meant by crossing a router in your answer???????? –  Zia ur Rahman Jul 23 '10 at 7:26
To change of network, you need a active equipment named router. This equipment send a packet to a destination IP address. If this IP address is private, it is not routable, so it will not cross the router –  Dom Jul 23 '10 at 9:59
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