What is the difference between public and private IP addresses?

If your intranet is not connected to the Internet, any IP addressing can be deployed. If direct (routed) or indirect (proxy or translator) connectivity to the Internet is desired, there are two types of addresses employed on the Internet, public addresses and private addresses .
 

Public Addresses:

Public addresses are assigned by InterNIC and consist of class-based network IDs or blocks of CIDR-based addresses (called CIDR blocks) that are guaranteed to be globally unique to the Internet.
When the public addresses are assigned, routes are programmed into the routers of the Internet so that traffic to the assigned public addresses can reach their locations. Traffic to destination public addresses are reachable on the Internet.

For example, when an organization is assigned a CIDR block in the form of a network ID and subnet mask, that [network ID, subnet mask] pair also exists as a route in the routers of the Internet. IP packets destined to an address within the CIDR block are routed to the proper destination.

Illegal Addresses:

Private intranets that have no intent on connecting to the Internet can choose any addresses they want, even public addresses that have been assigned by the InterNIC. If an organization later decides to connect to the Internet, its current address scheme might include addresses already assigned by the InterNIC to other organizations. These addresses would be duplicate or conflicting addresses and are known as illegal addresses . Connectivity from illegal addresses to Internet locations is not possible.

For example, a private organization chooses to use 207.46.130.0/24 as its intranet address space. The public address 207.46.130.0/24 has been assigned to the Microsoft corporation and routes exist on the Internet routers to route all packets destined to IP addresses on 207.46.130.0/24 to Microsoft routers. As long as the private organization does not connect to the Internet, there is no problem because the two address spaces are on separate IP internetworks. If the private organization then connected directly to the Internet and continued to use 207.46.130.0/24 as its address space, then any Internet response traffic to locations on the 207.46.130.0/24 network would be routed to Microsoft routers, not to the routers of the private organization.

Private Addresses:

Each IP node requires an IP address that is globally unique to the IP internetwork. In the case of the Internet, each IP node on a network connected to the Internet requires an IP address that is globally unique to the Internet. As the Internet grew, organizations connecting to the Internet required a public address for each node on their intranets. This requirement placed a huge demand on the pool of available public addresses.

When analyzing the addressing needs of organizations, the designers of the Internet noted that for many organizations, most of the hosts on the organization's intranet did not require direct connectivity to Internet hosts. Those hosts that did require a specific set of Internet services, such as the World Wide Web access and e-mail, typically access the Internet services through Application layer gateways such as proxy servers and e-mail servers. The result is that most organizations only required a small amount of public addresses for those nodes (such as proxies, routers, firewalls, and translators) that were directly connected to the Internet.

For the hosts within the organization that do not require direct access to the Internet, IP addresses that do not duplicate already-assigned public addresses are required. To solve this addressing problem, the Internet designers reserved a portion of the IP address space and named this space the private address space . An IP address in the private address space is never assigned as a public address. IP addresses within the private address space are known as private addresses . Because the public and private address spaces do not overlap, private addresses never duplicate public addresses.

The private address space specified in RFC 1918 is defined by the following three address blocks:

 

1. 10.0.0.0/8: The 10.0.0.0/8 private network is a class A network ID that allows the following range of valid IP addresses: 10.0.0.1 to 10.255.255.254. The 10.0.0.0/8 private network has 24 host bits that can be used for any subnetting scheme within the private organization.

 

2. 172.16.0.0/12: The 172.16.0.0/12 private network can be interpreted either as a block of 16 class B network IDs or as a 20-bit assignable address space (20 host bits) that can be used for any subnetting scheme within the private organization. The 172.16.0.0/12 private network allows the following range of valid IP addresses: 172.16.0.1 to 172.31.255.254.

3. 192.168.0.0/16: The 192.168.0.0/16 private network can be interpreted either as a block of 256 class C network IDs or as a 16-bit assignable address space (16 host bits) that can be used for any subnetting scheme within the private organization. The 192.168.0.0/16 private network allows the following range of valid IP addresses: 192.168.0.1 to 192.168.255.254.

The result of many organizations using private addresses is that the private address space is re-used, helping to prevent the depletion of public addresses.
Because the IP addresses in the private address space will never be assigned by the InterNIC as public addresses, there will never exist routes in the Internet routers for private addresses. Private addresses are not reachable on the Internet. Therefore, Internet traffic from a host that has a private address must either send its requests to an Application layer gateway (such as a proxy server), which has a valid public address, or have its private address translated into a valid public address by a network address translator (NAT) before it is sent on the Internet.