"A small company needs to connect four computers to Internet. Of course that they can use private IP addresses, but because these computers need to act as servers they need public IP addresses. The ISP has to assign them a class of IP addresses, the most appropriate one being a Class C. But because they use only four addresses it means that we will have 250 IP addresses lost in the cyberspace. To prevent such kind of situations, an IP address is used in conjunction with a network mask. Network administrators often need to divide a larger network into small networks called subnetworks or subnets."
FULL STORY @ PCHARDWARE (http://www.pchardware.ro/Articles/article.php?id=152)