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Notable 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, 171.15.85.0/24 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. Hi Rene,

    What do you mean by CIDR & VLSM ? The both are same or what is the difference between them.

    Br
    Zaman

  3. andrew says:

    Zaman,
    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 10.10.10.0/24 as an IP range. Without VLSM, you would have to pick a certain subnet mask, say, 255.255.255.240 (/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 10.10.10.248/30, for example, and with a subnet having 50 hosts, you could use 10.10.10.0/26, 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 192.168.0.0/24 and 192.168.1.0/24. These networks have a natural /24 boundary (because they are class C addresses), but I can using CIDR aggregate them via 192.168.0.0/23

  4. Thanks Andrew...,

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