Internet private addresses

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Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

Internet private addresses

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 places great stress on the available pool of public addresses.

To relieve this stress, IANA has provided for an address reuse scheme by reserving network IDs for private internetworks. 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. For more information, see A NAT example.

For more information about configuring NAT for the home office and small business, see SOHO Network to the Internet.