The routing prefix is /24 and that defines the subnet mask. You will frequently see CIDR notation this specifies the individual IP address or IP network along with a routing prefix. This method is known as Classless Inter-Domain Routing (CIDR), often pronounced as “cider.” How is this done? By changing the subnet mask to 255.255.255.0 instead of the default 255.0.0.0. (Breaking all the rules!) For example, one of my test networks uses the 10.254.254.0 network and the 255.255.255.0 subnet mask, making the network number 10.254.254 instead of just 10. This means that any network number can use any subnet mask. In fact, many corporate networks use classless IP addressing exclusively. While classful IP addresses used in Classes A, B, and C are still commonly implemented in SOHO networks, they are not quite as necessary any more in general. As such, it is interpreted by operating systems and routers as a group of Class C networks: 192.168.0, 192.168.1, 192.168.2, and so on… While this might appear to be class B mathematically (because of the /16), it is actually a set of 256 contiguous class C networks. Table 3: Private IP Ranges (As Assigned by the IANA) IP Class Most addresses, besides the private ones listed in Table 3, are considered public addresses. Public addresses are addresses that are displayed directly to the Internet they are addresses that anyone could possibly connect to around the world. A list of reserved private IP ranges is shown in Table 3. Typically, these are addresses that a SOHO router would assign automatically to clients. A private address is one that is not displayed directly to the Internet and is normally behind a firewall. It is also important to know the difference between private and public addresses. Net is the network portion of the IP address, whereas node is the host or computer portion of the address. Also, notice the 0 coincides with the name node. Notice in Table 2 how the number 255 in a subnet mask coincides with the name net. Table 2: IP Class Ranges and Their Equivalent Binary Values and Subnet Masks IP Class This is one of the reasons for the inception of IPv6. The total number of hosts, for all classes combined, is just under four billion-and we have pretty much used up all those addresses. This is because you can never use the first or the last address in the range the first is actually the network number and the last is the broadcast address. You might have also noticed that there are only 254 possible hosts per network in Class C (instead of 256). As mentioned previously, the built-in loopback address is 127.0.0.1. Technically, it is part of the Class A range, but it cannot be configured as an IP address within the IP Properties dialog box. That is because this network number is reserved for loopback testing. You probably noticed that the number 127 was skipped. Note: Class D (224 – 239) is used for multicast testing, and Class E (240 – 255) is reserved for future use.ĪPIPA addresses on the 169.254 network are part of Class B. If you have a small network at home, it is simplest and most common to use Class C. You realize that this classification system was created to appease different organizations of different sizes. Take a look at Table 1 and try to get a feel for the different IP classes available. Table 2 shows the IP classes and their associated default subnet masks, which, as we mentioned, identify which portion of the IP address is the network portion and which is the host portion. Table 1 shows the various classes and their associated IP address ranges. In that case, the first number is 192, which means that the IP address is part of a Class C network. When working with classful IPv4 addresses, the first number in the IP address dictates what class the address is part of. It’s embedded at the end of the article too.) (I also have a video about the differences between classful and classless here. But I do get questions from time to time on the subject, so if you are interested, read on… Because of this I have omitted most of the classful information from the upcoming A+ Exam Cram (to be released in 2019).
Most companies make use of a classless network (or CIDR), so classful IP is not as important anymore. I wrote this article to discuss IPv4 classes and classless addressing and compare the two.
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