title: IPv4 Classes - are they useful in any way nowadays?
body: <p>I'm trying to understand how an IPv4 address is structured, and which is the network, subnet and host part. So I've been reading Todd Lammle's "<strong>CCNA Routing and Switching</strong>" (Compete Study Guide, 2nd edition). As in many other networking books, the author here early on explains the various IP Classes.</p>
  
  <p>So there's <strong>Class A</strong> where the first octet of the address is 0 to 127, <strong>Class B</strong> from 128 to 191, <strong>Class C</strong> from 192 to 223, <strong>Class D</strong> from 224 to 239 and <strong>Class E</strong> from 240 to 255. There are also special addresses and ranges for private addresses, loopback addresses, broadcast etc. Moreover, apart from IP ranges, these Classes define which part of the address is the network and which is the node/host.</p>
  
  <p><a href="https://i.stack.imgur.com/4hdOm.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/4hdOm.png" alt="enter image description here"></a></p>
  
  <p>For instance, if an IP address begins with 200, then we know that it's a Class C and thus its first 3 bytes should define the network and the fourth one defines the host in that network.</p>
  
  <p>However, to my knowledge, <strong>classful routing is a thing of the past</strong> ever since CIDR was implemented decades ago. Thanks to CIDR we can use as many bits as we want for the network and host part. So for a Class C address we may use more than three bytes for the network e.g. <code>200.50.50.7 /26</code>, that way using 26 bits for the network and the rest for the host. I've also read that we can use <strong>less bits than the default</strong>! And thus, although a Class C address has 24-bit mask by default, we may use less than that e.g. <code>200.50.50.7 /16</code> (200.50 is the network in this case).</p>
  
  <p>But I was under the impression that Class C addresses - or in other words IPs between 192 and 223 - are supposed to use at least 24-bit masks...</p>
  
  <p>...or not??? Do Classes have any significance any more? Why do they teach us about them and mention them in books as if they are still relevant? <strong>Can we deduce anything about the IP address by looking at its first octet? Does the range tell us anything about the network and/or the host part of the address?</strong> For example, if an address begins with 130 (Class B) can I assume that at least the first two octets are used to define the network?</p>
  
  <p>In the aforementioned book "<strong>CCNA Routing and Switching</strong>", the author states that if an IP belongs to a certain Class, then it can only have specific CIDR values (have a look at the screenshot below).</p>
  
  <p><a href="https://i.stack.imgur.com/3YVRs.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/3YVRs.png" alt="enter image description here"></a></p>
  
  <p>That last part is rather confusing to me. Are IP ranges relevant even with CIDR?</p>