Private IP Address

InterNIC has reserved certain IP addresses as private addresses for use with internal web sites or intranets. These addresses are not routable on the public Internet, but are meant for devices that reside behind a router or other Network Address Translation (NAT) device or proxy server.

Private IP addresses are used either to hide systems from the public Internet or to provide an additional range of addresses to organizations that do not have sufficient public IP addresses to distribute on their network. Organizations can use these numbers to assign internal IP addresses without having to worry about an IP address conflict or having to obtain a new block of IP addresses.

If you connect to the Internet as a home user with a residential router, you will typically benefit from this arrangement. Although you may be paying for only one IP address through your Internet service provider (ISP), you can have unlimited devices connected to the Internet.

Using a private IP address will make your computer invisible to certain types of network attacks; however, you will not be able to easily establish your computer as a server.

Private Intranet addresses cannot be sent over the Internet. If an Internet router receives a packet with either a source or destination address that is reserved, it will drop the packet.

The following three blocks of the IP address space for private networks is reserved. Note that 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 255 contiguous class C network numbers.

1 × Class A
Class A network IP address range = 10.0.0.0 - 10.255.255.255
For one Class A network:
Subnet mask = 255.0.0.0
Network address length = 8 bit
Computer address length = 24 bit

16 × Class B
Class B network IP address range = 172.16.0.0 - 172.16.255.255
Class B network IP address range = 172.17.0.0 - 172.17.255.255
Class B network IP address range = 172.18.0.0 - 172.18.255.255

Class B network IP address range = 172.31.0.0 - 172.31.255.255
For each of the 16 Class B networks:
Subnet mask = 255.255.0.0
Network address length = 16 bit
Computer address length = 16 bit

Alternatively, 16 * Class B combined
Combined Class B networks IP address range = 172.16.0.0 - 172.31.255.255
For all 16 Class B networks combined:
Subnet mask = 255.240.0.0
Network address length = 12 bit
Computer address length = 20 bit

256 × Class C
Class C network IP address range = 192.168.0.0 - 192.168.0.255
Class C network IP address range = 192.168.1.0 - 192.168.1.255
Class C network IP address range = 192.168.2.0 - 192.168.2.255

Class C network IP address range = 192.168.255.0 - 192.168.255.255
For each of the 256 Class C networks:
Subnet mask = 255.255.255.0
Network address = 24 bit
Computer address = 8 bit

Alternatively, 256 × Class C combined
Combined Class C networks IP address range = 192.168.0.0 - 192.168.255.255
For all 256 Class C networks combined:
Subnet mask = 255.255.0.0
Network address length = 16 bit
Computer address length = 16 bit

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