IP Addressing

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: (hosts from to (hosts from to (hosts from to

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, 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 and a mask of (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 to 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…

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of 2000Trainers.com. He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.