We're Sorry, Full Content Access is for Members Only...

If you like to keep on reading, Become a Member Now! Here is Why:

  • Learn any CCNA, CCNP and CCIE R&S Topic. Explained As Simple As Possible.
  • Try for Just $1. The Best Dollar You've Ever Spent on Your Cisco Career!
  • Full Access to our 606 Lessons. More Lessons Added Every Week!
  • Content created by Rene Molenaar (CCIE #41726)


367 New Members signed up the last 30 days!


100% Satisfaction Guaranteed!
You may cancel your monthly membership at any time.
No Questions Asked!

Forum Replies

  1. Hi Rene,

    Question- does the subnet mask dictate the class of the network or the value of the octets? It seems here in this example that the subnet mask would dictate whether it was class A, B or C. However I was recently advised that it is not the subnet mask for example it has been recently stated to me:

    The class of a network is not determined by its subnet mask. The default subnet mask is determined by the class of a network. The class of a network is determined by the first octet of the IP.

    0-127 is considered “class A”.
    128-191 is considered “class B”.
    192-223 is considered “class C”.
    224-239 is considered “class D”.
    240-255 is considered “class E”.

    So, referring back to my previous example, is a “class B” address. The network mask which is used doesn’t matter. What matters is the first octet of the IP. In this case, since we are using a /24 mask, we have “subnetted” the class B address.

    I am looking for your insight on this. Thank you for your thoughts.

  2. James,
    Only the first four bits (3 really, since E class isn’t used) in the first octet determine class (as shown in the table below). You are correct in your example about

    Keep in mind, however, that “class” doesn’t really mean anything anymore. This is an outdated term that nobody really uses now. Even Cisco has stopped bothering to test on it!

    Binary (first 4 bits) Class
    0000                   A
    1000                   B
    1100                   C
    1110                   D
    1111                   E
  3. andrew says:

    These can be easily confused–in fact I had to review them myself to make sure I am giving you accurate information.

    VLSM is aptly named, because it means using a subnet mask of variable lengths throughout your organization with the same network block. For example, suppose your ISP has given you as an IP range. Without VLSM, you would have to pick a certain subnet mask, say, (/28) and stick with that. So if you wanted to use this block throughout your company, you would always have to use /28. This becomes wasteful in the case where you have a subnet with just two hosts, or it becomes inadequate where you have a subnet with 50 hosts. With VSLM, you have the freedom to change your subnet mask as needed, so in the case of a subnet with two hosts, you could use, for example, and with a subnet having 50 hosts, you could use, for example.

    CIDR is basically “supernetting.” This is where you can aggregate networks together into larger networks beyond their natural network boundary. For example, suppose I have and These networks have a natural /24 boundary (because they are class C addresses), but I can using CIDR aggregate them via

  4. Thanks Andrew…,

3 more replies! Ask a question or join the discussion by visiting our Community Forum