I'm not able to get what a private IP address is, and why is it used?

Though I searched for it in Google, I'm not able to get it. Can anyone explain me with an example?

  • What do you mean by, "I'm not able to get what a private IP...?" – Ron Maupin Jul 12 '17 at 16:38
  • I'm not able to understand what and why a private IP is used for. @RonMaupin – coderpc Jul 12 '17 at 16:39

I think your questions can be answered by looking at the RFCs. RFC 1918, Address Allocation for Private Internets defines private IP addressing.

You can make up your own addressing from any of the private IP address ranges defined in Section 3:

  1. Private Address Space


Section 2 gives the reason for private IP addressing:

  1. Motivation

















Basically, without private IP addressing, we would have run out of IPv4 addresses many years ago. There simply are not enough IPv4 addresses to for everyone to have a unique IPv4 address, which was the original premise of IP. To restore this premise and the end-to-end nature of IP, the IETF created IPv6.

  • Perfect ! Great information. – coderpc Jul 12 '17 at 16:52

Private IPv4 addresses allow you to run a network without applying for public IP addresses at your regional registry. Since IPv4 addresses have run out it's the only way to set up a new network or expand an existing one.

Private IPv6 addresses allow you to design your network in such a way that some entirely local services simply can't communicate with the outside world.


(note: this answer limits itself to IPv4, the IPv6 side of things is interesting but it's drifting a bit far from the question topic)

The original intent of private addresses was to allow people to run networks that didn't directly exchange packets with the global Internet without needing to assign global addresses to all machines on those networks. This saved addresses, and reduced the likelihood of inadvertant communication between those hosts and the outside works.

Later NAT came along. This allowed hosts with private addresses to make outgoing TCP/UDP connections to hosts on the public Internet. The NAT would replace the address and port used by the client with a globally reachable address and a suitable port (which may or may not be the same as the port the client originally used) and keep a mapping table so that replies could be translated.

Many companies found the advantages of running private IP addresses with NAT compelling. There was less possibility of inadvertantly exposing stuff to the whole world, no need to justify address usage, no need to pay per-address fees that some ISPs charged and for smaller companies no need to readdress their systems when changing ISP (for larger companies the final point was not an issue because they could get provider-independent address allocations).

In the home and mobile sectors too NAT became common. In the home case NAT became ubiquitous with the move from plain modems to "home routers". In the mobile case NAT was implemented at the ISP level.

So nowadays the vast majority of end-user systems use a private IP address behind a NAT.

  • So, the reasons for the creation of private addressing given in the RFC for private addressing do not meet your approval? As it says, the real motivation was the growth of the Internet routing table: "A separate and far more pressing concern is that the amount of routing overhead will grow beyond the capabilities of Internet Service Providers." – Ron Maupin Jul 12 '17 at 20:51

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