Private vs Public Addresses

Rapid growth of the Internet resulted in a shortage of IPv4 addresses. So, the powers that be designated a specific subset of the IPv4 address space to be private, to temporarily alleviate this problem.

A public address can be routed on the Internet. Thus, devices that should be Internet accessible (such as web or email servers) must be configured with public addresses.

A private address is only intended for use within an organization, and can never be routed on the internet. Three private addressing ranges were allocated, one for each IPv4 class:

Class A - 10.x.x.x

Class B - 172.16-31.x.x

Class C - 192.168.x.x

NAT (Network Address Translation) is used to translate between private addresses and public addresses. NAT allows devices configured with a private address to be stamped with a public address, thus allowing those devices to communicate across the Internet.

NAT is only a temporarily solution to the address shortage problem.

Eventually, IPv4 is about to be completely replaced with IPv6.

Two other ranges, while not considered “private,” have been reserved for specific use:

127.x.x.x - reserved for diagnostic purposes. One such address

(127.0.0.1), identifies the local host, and is referred to as the loopback or localhost address.

169.254.x.x - reserved for Automatic Private IP Addressing (APIPA).

A host assigns itself an APIPA address if a DHCP server is unavailable to dynamically assign an address.