I mentioned private IP addresses in the previous paragraph. These are ranges of IP addresses that have been reserved for internal usage on private networks and are not routable on the Internet. The private ranges of IP addresses are often misconceived to be classful, when in fact they are classless. The private ranges of IP addresses, as defined in RFC 1597 are:
10.0.0.0/8 (hosts from 10.0.0.1 to 10.255.255.254)
172.16.0.0/12 (hosts from 172.16.0.1 to 172.31.255.254)
192.168.0.0/16 (hosts from 192.168.0.1 to 192.168.255.254)
Since these ranges are private and can be used by anyone, they can be further subnetted to meet the needs of an organization. Note that Windows 2000 also uses another range, 169.254.0.0/16 for automatic private IP addressing (APIPA).
After all that, what I’m trying to get across to you is that the subnet mask helps us better understand the context of an IP address. You need to recognize that on its own, the IP address tells us nothing of how the network is configured. If I tell you I have a host with an IP address of 126.96.36.199 and a mask of 255.255.248.0 (which works out to /21), then I can also tell you that local hosts (or hosts on this same network/subnet) range in IP address from 188.8.131.52 to 184.108.40.206. If the range above was provided to me by my ISP, I could still subnet it further to meet my own subnetting needs. More on the details of how to subnet later…