Private IP Addresses

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Private Addresses

There are certain addresses in each class of IP address that are not assigned. These addresses are called private addresses. Private addresses might be used by hosts that use network address translation (NAT), or a proxy server, to connect to a public network; or by hosts that do not connect to the Internet at all.

Many applications require connectivity within only one network and do not need external connectivity. In large networks, TCP/IP is often used, even when network layer connectivity outside the network isn’t needed. Banks are good examples. They may use TCP/IP to connect to automatic teller machines (ATMs). These machines do not connect to the public network, so private addresses are ideal for them. Private addresses can also be used on a network where there are not enough public addresses available.

The private addresses can be used together with a network address translation (NAT) server. Either a NAT server or a proxy server to provide connectivity to all hosts in a network that has relatively few public addresses available. By agreement, any traffic with a destination address within one of the private address ranges will NOT be routed on the Internet.

Available Address Ranges

Class A; –
* Class B; –
Class C; –

  • There are three classes of IP addresses that an organization can receive from InterNIC: Class A, B, and C.
  • InterNIC reserves Class A addresses for governments throughout the world, Class B addresses for medium-size companies, and Class C addresses for all other entities.
  • When written in a binary format, the first bit of a Class A address is always 0.
  • The first 2 bits of a Class B address are always 10, and the first 3 bits of a Class C address are always 110.

* The Internet Assigned Numbers Authority (IANA) has reserved through for Automatic Private IP Addressing. Windows 98, Millennium, 2000, and XP support APIPA.

  • Windows 2000 and Xp TCP/IP attempts to find a DHCP server on the attached network to obtain a dynamically assigned IP address.
  • In the absence of a DHCP server during startup (for example, if the server is down for maintenance or repairs), the client cannot obtain an IP address.
  • APIPA generates an IP address in the form of 169.254.x.y (where x.y is the client’s unique identifier) and a subnet mask of If the address is in use APIPA selects another IP address and if necessary, reselects addresses up to 10 times. After the computer generates the address, it broadcasts to this address and then assigns the address to itself if no other computer responds. The computer continues to use this address until it detects and receives configuration from a DHCP server. This allows two computers to be plugged into a LAN hub, to restart without any IP address configuration, and to use TCP/IP for local network access.