Private IP address

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(Redirected from RFC 1918)

Private IP address space has been allocated via RFC 1918. This means the addresses are available for any use by anyone and therefore the same private IP addresses can be reused. However they are defined as not routable on the public Internet. They are used extensively in private networks due to the shortage of publicly registerable IP addresses and therefore network address translation is required to connect those networks to the Internet. IPv6 was also created to alleviate this shortage, but is not in widespread use yet.

Private IP addresses also provide a basic form of security as in a typical network configuration of this type it is not possible for the outside world (Internet) to establish a connection directly to a host using these addresses.

Specifically the networks are:

Name IP address range number of IPs classful description largest CIDR block
24-bit block 10.0.0.0 – 10.255.255.255 16 777 215 single class A 10.0.0.0/8
20-bit block 172.16.0.0 – 172.31.255.255 1 048 576 16 contiguous class Bs 172.16.0.0/12
16-bit block 192.168.0.0 – 192.168.255.255 65 535 256 contiguous class Cs 192.168.0.0/16

RFC 1597 was the original specification but is now for historical purposes only.

To reduce load on the root nameservers caused by reverse DNS lookups for these IP addresses, a system of "black-hole" nameservers is provided by anycast network AS112. [1] (http://www.as112.net/)

Bit-wise representation

...
 start @  10   0.  0.  0 = 00001010.00000000.00000000.00000000 (private subnet mask)
finish @  10.255.255.255 = 00001010.11111111.11111111.11111111 (private broadcast address)
                           00001010.XXXXXXXX.XXXXXXXX.XXXXXXXX (private 24 bit block)
...
 start @ 172. 16.  0.  0 = 10101100.00010000.00000000.00000000 (private subnet mask)
finish @ 172. 31.255.255 = 10101100.00011111.11111111.11111111 (private broadcast address)
                           10101100.0001XXXX.XXXXXXXX.XXXXXXXX (private 20 bit block)
...
 start @ 192.168.  0.  0 = 11000000.10101000.00000000.00000000 (private subnet mask)
finish @ 192.168.255.255 = 11000000.10101000.11111111.11111111 (private broadcast address)
                           11000000.10101000.XXXXXXXX.XXXXXXXX (private 16 bit block)
...

See also

External links

it:Indirizzo IP privato nl:RFC 1918

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