Every machine that is directly connected to the Internet must have a public IP address, commonly known as a routable address. A routable address is one that a connection can be made to from anywhere on the TCP/IP network — in this case, the Internet. For example, any web site you visit that is on the Internet has a routable address. If it were non-routable, packets would not be able to be routed to it. Each IP address class has its own non-routable address (they cannot be routed on the Internet), which can be used in a private IP network (one that is not on the Internet). Non-routable addresses are commonly used in an organization or a home network that is not directly connected to the Internet. It is customary (and cost effective, as routable IP addresses cost money!) to have a Network Address Translation (NAT) box that acts as a gateway to the Internet for your non-routable addresses.

There is one very special address that you will find on every TCP/IP host, and that is The address is commonly referred to as the loopback address and is a virtual network that exists only on your local machine. The loopback address is used for testing a TCP/IP network and is useful if you want to test whether or not your network services are working. It also helps any process that needs to communicate over TCP/IP to a service locally on the machine because that process can use the loopback address. The loopback address is not linked to a physical network device, but to a logical lo (loopback) device on your system. If you type ifconfig on the command line of your SUSE host, you will see the loopback device listed with an address of

As each class of IP network has its own non-routable address space, you can base how you would use those private addresses in your organization (or at home) on how network assignments work in the routable space of that class.

Non-Routable Classed Networks
Class Non-Routable Addresses


If your organization needed a flat IP address space, you could assign a non-routable Class A address range to all of your internal machines. However, this is usually wasteful and a nightmare for the network manager because there is no logical distinction between departments or machine use. One way to combat this is via subnetting.

It is common that if you have a small to medium organization, you can set up your network. This would use the networks,,, and As these are using a subnet mask of (the default for a Class C network), these networks are seen from a networking standpoint as being separate entities.