A tutorial on IP network addressing
Jan 1, 2006 12:00 PM, By Brad Gilmer
Network addresses
Now back to the question: Why would the number /29 be chosen to represent eight addresses on our network? To answer this question, it might be useful to look at the network address assigned by the ISP and its corresponding subnet mask in binary. (See Tables 3 and 4.)
If you count the number of “ones” in the binary representation of the subnet mask, you will find that there are 29 of them. (Ah ha!) Not only that, if you look at the subnet mask, you will see that only three binary numbers are set to zero — the last three. A binary value of 111 equals seven. So, if you use the subnet mask to strip off all but the last three digits of the assigned IP address, the maximum number of values that can be represented is eight (zero through seven).
As you can imagine, it is possible to design logic that can quickly strip off the first 29 bits, or the last three bits. Why would this be important? Because in Internet technology, it can be important to know what addresses are local and what addresses are network. Put in other terms, it may be important to determine whether traffic on the network is meant for a local host or for the Internet.
 Table 4. A network address’ corresponding subnet mask in binary numbers. Click here to see an enlarged diagram. |
One reason CIDR notation is common is that it is convenient. It is much easier to say “66.235.22.8/29” than it is to say “66.235.22.8 with eight valid IP addresses,” or “66.235.22.8 with a subnet mask of 255.255.255.240.”
Setting the subnet mask
Finally, you may wonder if it is a problem to leave your computers set to a subnet mask of 255.255.255.0. That depends on your situation. If you are working with host computers connected to the Internet, it would be best to set the subnet mask correctly. If you are dealing with workstations connected to a private network, then setting the subnet mask correctly is not as critical.
To learn more about IP addressing, take a look at an article titled “IP addressing and subnetting for new users,” available at www.cisco.com, document number 13788. You may also want to download a free advanced subnet calculator from www.solarwinds.com. The calculator not only does subnet calculations, but also converts between CIDR and conventional subnet notation as well as performs other useful tasks.
Brad Gilmer is a SMPTE Fellow. He is also president of Gilmer & Associates, executive director of the Video Services Forum and executive director of the AAF Association.
Send questions and comments to: brad.gilmer@penton.com
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