Internet Private Addresses

Applies To: Windows Server 2008

To communicate on the Internet, you must use addresses that have been allocated by the Internet Assigned Numbers Authority (IANA). Addresses allocated by IANA can receive traffic from Internet locations and are known as public addresses. A typical small business or home office is allocated a public address (or addresses) from its Internet service provider (ISP), who has received a range of public addresses.

To allow multiple computers in the small office or home office to communicate on the Internet, each computer must have its own public address. This requirement creates great demand for public addresses, which are finite in number.

To satisfy this demand, IANA has provided for an address reuse scheme by reserving network IDs for private networks. The private network IDs include:

  • 10.0.0.0 with the subnet mask 255.0.0.0

  • 172.16.0.0 with the subnet mask 255.240.0.0

  • 192.168.0.0 with the subnet mask 255.255.0.0

For more information about portions of the IP address space that are reserved for private intranets, see RFC 1918, "Address Allocation for Private Internets." All addresses in these ranges are known as private addresses.

Private addresses cannot receive traffic from Internet locations. Therefore, if an intranet is using private addresses and communicating with Internet locations, the private address must be translated to a public address. A network address translator is placed between an intranet that uses private addresses and the Internet, which uses public addresses. Outgoing packets from the intranet have their private addresses translated by network address translation (NAT) into public addresses. Incoming packets from the Internet have their public addresses translated by NAT into private addresses.

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