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Private Address space is used when IP addresses are needed in a private
LAN that is also connected to the public Internet. For example in an office environment
or in a data centre, where backend servers are used (such as transaction servers).
For this reason three ranges have been assigned for this purpose by
the Internet Assigned Numbers Authority (IANA). See
RFC 1918 for further details.
The 3 ranges are:
| From |
To |
Range |
| 10.0.0.0 |
10.255.255.255 |
(10/8 prefix) |
| 172.16.0.0 |
172.31.255.255 |
(172.16/12 prefix) |
| 192.168.0.0 |
192.168.255.255 |
(192.168/16 prefix) |
We will refer to the first block as "24-bit block",
the second as "20-bit block",
and to the third as "16-bit" block.
Note that (in pre-CIDR notation) the first block is nothing but a single class A
network number, while the second block is a set of 16 contiguous
class B network numbers, and third block is a set of 256 contiguous
class C network numbers.
These private addresses are never routed by the public Internet and so are much more secure
for LAN's. They are also often used in NAT (Network Address Translation)
devices such as firewalls for added security.
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