TCP/IP Address Classes & Reserved IP Space

Since IP addresses consist of 32 bit numbers, that means there are 232 available IP addresses on the Internet. 232 = 4,294,967,296 addresses between and The powers that allocate IP addresses arbitrarily broke up IP space into the network classes A,B, & C. They allocate these subnets to different organizations according to an organization's estimated needs. Those organizations then dole out portions of their IP space to subscribers or even other smaller Internet Service Providers.

Here are the traditional classful network sizes and boundaries[1]:

Table 1-4. IP address classes

Classaddress rangenetwork/host parts# of networks# of hosts
You have undoubtedly noticed a couple of networks missing from this. The class A sized network is reserved for localhost and loopback testing. Addresses are class D multicast and class E experimental addresses. We won't be discussing multicast here, of course.

There are also specific networks described as private IP space. These networks are reserved from the public IP classes described above. Private IP space can be used by networks not connected to the Internet directly. There is no fear over IP address overlap because these private blocks are not intended to be routed across the internet[2].

You've probably already noticed our use of addresses from these private IP blocks in our examples. These are the addresses you'll be using in your private home networks while you probably only get 1 public IP address from your Internet Service Provider.

You probably also noticed the odd notation,, and What is /8, /12, and /16? The next section explains that we can subnet in many other ways besides breaking on traditional 8-bit boundaries....



The original IP address classes are described in RFC 1020 published way back in 1987. RFC's, Requests For Comments are the documents that define the internet standards we live by. There are thousands of them and the body of them represents both the current state of internetworking as well as the evolution of the net over time. The very name indicates the open and voluntary nature of the internet. Visit to learn more about how they work.


These blocks are officially defined in RFC 1918.