Public, Private, and APIPA IP Addresses
An IP address visible to any computer or device on the Internet is a public IP address. If you use a web service to display your computer’s IP address (see Figure 2-5), the public IP address is the one that is displayed.
Figure 2-5Using the WhatIsMyIPAddress.com website to determine the public IP address for acomputer.
However, computers that are connected to the Internet via a business or a home network are actu-ally using a private IP address that is translated into a public IP address.
Each of the classes listed earlier in Table 2-3 includes a range of private IP addresses. A private IP address is not visible on the Internet, although devices with a private IP address can connect to the Internet.
The private IPv4 address ranges for classes A, B, and C are as follows:
Class A—10.0.0.0–10.255.255.255
Class B—172.16.0.0–172.31.255.255
Class C—192.168.0.0–192.168.255.255
To see the IP address assigned to your computer, open a command prompt and use the command-line IPCONFIG.EXE utility. Figure 2-6 shows the network-assigned IPv4 address for the computer whose public IP address was shown earlier in Figure 2-5. As you can see, the computer is using
a private IPv4 address. The router translates this address into the public IP address shown in Figure 2-5.
46 CompTIA A+ Quick Reference
Figure 2-6Output fromIPCONFIG /ALLlisting IP addresses, gateways, and DNS servers for awireless network connection.
When a computer or device on a private network needs to connect to the Internet, a router or gate-way makes the connection and uses a feature called Network Address Translation (NAT) to con-vert the private IP address into a public one. The conversion process happens in reverse when the information is relayed back to the requesting device on a private network.
Private IP addresses are usually assigned by a DHCP server. A DHCP server can be a routine run-ning on a server or a feature of a router or gateway device.
In the event that a computer cannot receive an address via DHCP and does not have a fixed IP address, Windows uses a features known as Automatic Private IP Addressing (APIPA) to assign an IPv4 address in the 169.254.x.x address range. This address range does not support Internet access, but enables computers on a local-area network (LAN) to share folders and printers.
In IPv6, the equivalent to an APIPA address is a link-local address. IPv6 can be routed over IPv4 (this enables both versions of IP to coexist), so as long as the computer also has a valid IPv4 address that can connect to the Internet through a gateway, a computer with a link-local IPv6 address will have Internet access (refer to Figure 2-6).