Private Addresses
Private addresses are special addresses set aside by IANA for use within networks that will not be seen by the Internet. These private addresses are private because only the network that uses them should see them and they should not be 'seen' by the Internet. These addresses cannot be used on the Internet or to communicate with the Internet. Many ISP's filter out and delete packets using private IP addresses. Any organization that uses private IP addresses on computers that communicate with the Internet must use a device that performs Network Address Translation (NAT).
Anyone can use private addresses and they don't need to consult or ask permission from the Regional Internet Registries. So long as the network using private IP addresses does not communicate directly with the Internet using those private addresses, there is no problem. However, anyone can use private IP addresses and because IP addresses must be unique, networks using private addresses should not talk to each other, or to the Internet unless they are using NAT or NAT with overload (Port Address Translation or 'PAT').
There are several blocks of private addresses that were set aside by IANA specifically for this purpose.
- Private Class A range- 10.0.0.0 - 10.255.255.255
- Private Class B range -172.16.0.0 - 172.31.255.255
- Private Class C range - 192.168.0.0 - 192.168.255.255
- Loopback Addresses - 127.0.0.0-127.255.255.255
The Class A range provides for up to 16,777,214 hosts on one network. In practice, most organizations that use the Class A range of addresses usually subnet this range of addresses into smaller sets of hosts called subnetworks (or just 'subnets' for short). Still, even with the subnetting, it makes for a conveniently large number of addresses.
The Class B range of IP addresses fall into 16 subnets. These were provided so that an organization could have up to 65,534 hosts on a network and have up to 16 networks.
The Class C range of IP addresses is designed to support 254 networks of up to 65,534 hosts.
The Loopback address range was set aside for what is called loopback testing. Loopback testing is the process of testing the IP networking software protocol stack and the hardware by pinging the address 127.0.0.1. Only a single IP address was needed for this purpose but an address range of 16,777,214 was set aside for this purpose. At the time this was set aside, there were very few computers and 127 was an easy bit pattern to recognize (0111111). Usually, pinging any IP address in this range produces the same result:
Microsoft Windows XP [Version 5.1.2600] C:\Documents and Settings\InetDaemon>ping 127.1.1.1 Pinging 127.1.1.1 with 32 bytes of data: Reply from 127.0.0.1: bytes=32 time<1ms TTL=128 Ping statistics for 127.1.1.1: |
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