Public and Private IP addresses

On the Internet, a limited number of IP addresses exist. We have already mentioned IP address conflicts in the previous chapter. Each computer that wants to communicate on a network needs a unique IP address to function.

This is also true for the Internet in general. All IP addresses in use must be unique and must not be used anywhere else on the Internet.

However, there are exceptions to this rule. One such exception is all IP addresses beginning with “192.168” which belong to a special category of IP addresses that are commonly called Private IP addresses. These Private IP addresses are reserved for local use within LAN networks. They can be reused in many places. Actually, you will find IP addresses from the 192.168. address range at almost any home and even at many corporate networks.

Two other such Private address spaces exist

• Any address beginning with “10.”
• All addresses beginning with “172.16” up to “172.31” for example 172.16 or 172.23 or 172.31

Most often home routers use IP networks that begin with “192.168” on the inside LAN. The two absolutely most commonly used IP networks on the inside LAN of home routers are “192.168.0.0” and “192.168.1.0” with the Subnet Mask 255.255.255.0

As mentioned earlier, the above special addresses are called Private addresses. They cannot be used on the Internet, they can only be used within local networks. If you try to use Private addresses on the Internet then your Internet Service Provider will block your traffic automatically, sensing that the traffic is coming from a Private IP address. This automatic block is being done to avoid any IP address conflicts on the Internet. These addresses are used in so many places that without the block we would have guaranteed and constant IP address conflicts all over the Internet.

So how can the home network function and how can you browse the Internet from a computer which is configured with one of these Private IP addresses? Obviously, almost everybody is using such private addresses, so what is preventing those constant IP address conflicts?

The secret is in something called Address Translation, or NAT (Network Address Translation) which is discussed in the next section.