General Information

All of the hosts that are connected to the Internet must have a unique public IP address. There is a risk that IP addresses could be ran out because 32-bit address space is limited. One possible approach to solve this problem is to keep some private address for internal use only, so that the organization without public IP address can still communicate with each other.

RFC 1918 standard reserves some address range in A, B and C classes respectively. They include a private address range of Class A network, 16 Class B networks and 256 Class C networks, allowing network administrators to assign the internal address in great flexibility.

The large networks can use the Class A private network address, which provides over 16 million private addresses. Medium-sized private network can use Class B network address, which provides more than 65,000 addresses. Home and small business networks often use a class C private address, which can accommodate up to 254 hosts. The last one is commonly used in home networks, and the range could be starting from, or etc.

The organization of any size can use a Class A network, 16 Class B networks or 265 Class C networks which are defined by RFC 1918. Typically, many organizations use Class A private network, because it provides enough addresses for the hosts in the organization.

As long as the host is not directly connected to the Internet, they can use private addresses. Thus many organizations can use the same set of private addresses. Private addresses can not be routed in to the Internet, because they will be blocked by the router of ISP. There is also another technology called NAT to translate these private addresses to public ones and then the computers could be indirectly connect to Internet.

Private address is only visible in the local network and external personnel can’t access the private IP address directly, so it is more security than public ones.
There are a number of private addresses can be used for equipment testing, such private addresses is called the loopback address. A class A network is reserved as loopback addresses. Microsoft has also reserved an address range for Automatic Private IP Addressing(APIPA). Using APIPA, if the DHCP server is not available, DHCP client will automatically configure to their own IP address and subnet mask. The IP address range that is used for APIPA is ~ The client would give his address at the range and the default Class B subnet mask of Client has been configured to use their own IP address, until the DHCP server is available.