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Public and private IP addresses

IP addresses can be public or private. Public IP addresses are addresses recognized across the Internet. When you register with InterNIC, they issue you a public IP address, which ensures that the address is unique. However, IP addresses, like phone numbers for telephones, are becoming scarce. As a result, networks are using private IP addresses, which are valid only on a local intranet. It is never used on the Internet. The technique used for private addressing is called subnetting, and will be discussed in the next lesson. It is not necessary to register private IP addresses.

Difference between public and private IP Addresses

A public IP address is an IP address that can be accessed over the Internet. Like a postal address used to deliver a postal mail to your home, a public IP address is the globally unique IP address assigned to a computing device. Your public IP address can be found at What is my IP Address page.Private IP address on the other hand is used to assign computers within your private space without letting them directly expose to the Internet. For example, if you have multiple computers within your home you may want to use private IP addresses to address each computer within your home. In this scenario, your router get the public IP address, and each of the computers, tablets and smartphones connected to your router (via wired or wifi) get a private IP address from your router via DHCP protocol.
(IANA) Internet Assigned Numbers Authority is the organization responsible for registering IP address ranges to organizations and (ISPs) Internet Service Providers . To allow organizations to freely assign private IP addresses, the Network Information Center (InterNIC) has reserved certain address blocks for private use. The following IP blocks are reserved for private IP addresses.

Networks behind Router

Small organizations with limited Internet gateway access can place their networks behind a router that keeps the addresses private and use the private IP addresses that are designed for this purpose. Private IP addresses are not recognized as valid on the Internet. The source address of IP packets originating on the private network is modified to conform to the public IP address. Conversely, IP packets from the outside are readdressed with the private destination address when they pass through the router from the outside into the private network. There are two difficulties with this approach:
  1. Traffic passing through the router to the outside, for exampleWeb requests, must be carefully tracked by the router, so that responses from the outside are routed to the correct private address on the inside. For a small network, this is relatively manageable. The task becomes far more challenging when the number of privately addressed computers is large and traffic through the interface is substantial. The correct routing of e-mail is particularly challenging.
  2. An organization with multiple local area networks connected with a backbone network must configure the private networks in such a way that traffic between the various private local area networks can be managed successfully. One possible solution is to use a single private IP addressing scheme for all networks attached within the backbone, with translation at the edge router. Again, the problem of translation becomes large and difficult to manage.