Subnet Addressing

By Ron Cooney

Questions regarding this article should be sent to the author at ron.cooney@paranet.com .

The issue of subnetting has long been a mystery to many system administrators. It seems that there are just too many bits, bytes, and masks involved to make it worthwhile. Besides, who needs all that trouble when it's all you can do to just maintain the network as it is? You can certainly find sympathy, but if your network plans call for Internet access then IP (Internet Protocol) addressing and possibly subnet masking are topics that you s hould be familiar with.

As networks grow to increasing numbers of segments, more network address numbers will be needed as each segment requires an individual number. The InterNIC (whose name signifies cooperation of Network Information Centers or NICs), however, is not eager to hand out unlimited network addresses because they are quickly running out of them. The Internet community is generally taking a firm stand on limiting address availability, due to the accelerated growth demand for Internet access. Network administrators will have to work with what they have to better accommodate network requirements and the shrinking supply of address numbers.

One way of accomplishing this address conservation is to take the address that is assigned to your network and expand the capacity with subnets. Subnetting allows you to increase the number of networks available to you without applying for another IP address number. IP Addressing

The IP addressing framework allows one to address about 16,000,000 unique hosts for a Class A address, around 65,000 hosts for a Class B address, but only 254 hosts for a Class C address. However, there are no more Class A addresses available, and the InterNIC has stopped assigning Class B addresses. Class C addresses are the most numerous, but their limitation is that each can identify only 254 unique hosts.

The IP address is composed of 32 bits, which consist of two parts: the most significant bits (MSBs) identify a particular network and the remaining bits specify a host on that network. The most significant bits of the network portion actually determine the address class as shown in this table:

Address MSB Class Pattern

Class A 0 Class B 10 Class C 110

Class A Address Example

A class A address could be diagramed

Network Host

• ------+ +----------------------+

| | | |

0xxxxxxx?

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