8 Steps to Understanding IP Subnetting

Step 5 - Public Vs. Private IP Addresses

Technically, if all the possible combinations of IP addresses were available, there would be about 4,228,250,625IP addresses for use. This would have to include all public uses and private uses - which would then mean, by definition, there would be nothing but public IP addresses.

However, not all addresses are available. Some are used for special purposes. For example, any IP address ending in 255 is a special broadcast address.

Other addresses are used for special signaling, including:

  • Loopback ( when a host is referring to itself
  • Multicast routing mechanisms
  • Limited broadcasts sent to every host, but limited to the local subnet
  • Directed broadcasts first routed to a specific subnet, and then broadcast to all hosts on that subnet

The concept of a private address is similar to that of a private extension in an office phone system. Someone who wants to call an individual in a company would dial the company’s public phone number, through which all employees can be reached. Once connected, the caller would enter in the extension number of the person to whom they wished to speak. Private IP addresses are to IP addresses what extension numbers are to phone systems.

Private IP addresses allow network administrators to extend the size of their networks. A network could have one public IP address that all traffic on the Internet sees, and hundreds - or even thousands - of hosts with private IP addresses on the company subnet.

Anyone can use a private IP address on the understanding that all traffic using these addresses must remain local. It would not be possible, for example, to have an email message associated with a private IP address to move across the Internet, but it is quite reasonable to have the same private IP address work well in the company network.

The private IP addresses that you can assign for a private network can be from the following three blocks of the IP address space:

  • to Provides a single Class A network of addresses
  • to Provides 16 contiguous Class B network addresses
  • to Provides up to 216 Class C network addresses

A typical network setup using public and private IP addresses with a subnet mask would look like:

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Written by Dale Janssen
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Dale Janssen is a co-founder of Techopedia and has been involved in the IT Industry for most of his career. He purchased his first computer in 1978 - an Ohio Scientific computer with a whopping memory size of 48K, operating a relic of an operating system called CPM. In fact, his roots go so far back in the computing biz that when he graduated from university, there was no such thing as a computer science department.