Learning subnet, classful/classless, etc.



  • Hi everyone :).

    I use Pfsense, and currently I am also in school learning more computer and network skill. Subnetting has always been a mystery to me, so that is not something I've used. But I feel quite proud that I think I figured out the most basics of it now, how to calculate how many subnets.. valid subnets.. ranges.. to where a IP belongs.. binary/decimal.. In the end this will help me in small Cisco exam.
    I made a little chart thing on my own.. See Attachment. one of many.

    Question 1.
    Is there anything really wrong with that chart I made?. I feel like something is wrong.. I feel there is multiple names of everything.

    Question 2.
    Is CIDR the same as "Slash notation"?.  isn't it always a CIDR when there is.. "/14" … "/24" ?. last one not a CIDR?.

    Question 3.
    So.. ….  255.255.255.0  cannot be called a subnet?, only when it has the new extra bits in a octet?. Right?... because it is not divided.

    11111111.11111111.11111111.00000000  Not a subnet.
    11111111.11111111.11111111.11000000  To be called a subnet with "Subnet bits"?.

    Question 4.
    Are all these "Classful" ?.
    255.0.0.0
    255.255.0.0
    255.255.255.0

    And anything in between is Classless?. For some reason i want to understand it like that, but no place is it written as simple as that.

    Question 5.
    Can any IP have any Mask?.

    ![2018-02-04 20_49_25-octet to decimal - Excel.png](/public/imported_attachments/1/2018-02-04 20_49_25-octet to decimal - Excel.png)
    ![2018-02-04 20_49_25-octet to decimal - Excel.png_thumb](/public/imported_attachments/1/2018-02-04 20_49_25-octet to decimal - Excel.png_thumb)



  • 255.255.255.0 is just a subnet mask. You'd use this mask in conjunction with an address space like 10.0.0.0 or 192.168.1.0 or 4.4.4.0. You apply the mask against the IP address and you get the subnet. I do not work in networking, but I have worked with networking and I've never heard anyone use the term "Classful" in the real world. I've never heard anyone say "Class A" network, they just say "/8".



  • @Harvy66:

    255.255.255.0 is just a subnet mask. You'd use this mask in conjunction with an address space like 10.0.0.0 or 192.168.1.0 or 4.4.4.0. You apply the mask against the IP address and you get the subnet. I do not work in networking, but I have worked with networking and I've never heard anyone use the term "Classful" in the real world. I've never heard anyone say "Class A" network, they just say "/8".

    Hehe okay, thanks :). Funny how there are some terms you just don't find outside school, in almost any trade.. but somebody must use it!.



  • First off, classful addressing is obsolete.  No need to worry about it, except for history. CIDR was developed for more efficient use of the address space.
    2nd, there's no difference between the notations, other than how they're written down.  They both mean the same thing.  So, 255.255.255.0 is the same as /24.  The slash format is now the standard method.
    Again, "mask" form is obsolete, replaced by the / form.
    In Q4, they could be classful A, B & C, but again, we no longer worry about classes, only how much of the address is network vs host address.
    Q5, no, currently only contiguous strings of 1 bits, starting from the left are allowed, however, way back in the dark ages, any pattern was legal.
    Q3, any contiguous block of addresses is a subnet, with the smallest being a /31 or 2 addresses.  This would be used on point to point links.

    At first glance, the chart appears OK.

    Now, on to 128 bit IPv6 addresses…  ;)



  • @JKnott:

    First off, classful addressing is obsolete.  No need to worry about it, except for history. CIDR was developed for more efficient use of the address space.
    2nd, there's no difference between the notations, other than how they're written down.  They both mean the same thing.  So, 255.255.255.0 is the same as /24.  The slash format is now the standard method.
    Again, "mask" form is obsolete, replaced by the / form.
    In Q4, they could be classful A, B & C, but again, we no longer worry about classes, only how much of the address is network vs host address.
    Q5, no, currently only contiguous strings of 1 bits, starting from the left are allowed, however, way back in the dark ages, any pattern was legal.
    Q3, any contiguous block of addresses is a subnet, with the smallest being a /31 or 2 addresses.  This would be used on point to point links.

    At first glance, the chart appears OK.

    Now, on to 128 bit IPv6 addresses…  ;)

    Thank you :)

    Ok, it is obsolete!. I also read that today, that it changed in the 90's.
    I guess it also make sense then why I can't find that many articles about classful/classless, or atleast where it is clearly defined as id expect.. if it doesn't matter that much anymore. All the time I have looked for some hardcore rule saying you can't make classful/classless with X or Y CIDR/ or IP. But it may still be something I need to know.. for history and comparison.
    I will read more about this obsolute thing and worry less :P.

    Would you rephrase Question 3 answer for me ? :), and yes /31 is a special case.

    For example… the typical 192.168.1.0/24    .. would you still call that a subnet even thought there only is those 254 host adresses, not divided or anything.



  • Would you rephrase Question 3 answer for me ? :), and yes /31 is a special case.

    For example… the typical 192.168.1.0/24    .. would you still call that a subnet even thought there only is those 254 host adresses, not divided or anything.

    The /24 means that 24 bits are used for the network and 8 for the hosts.  That's a contiguous block of 256 addresses, with "0" the network address and "255" for the broadcast address on that subnet.  A mask always provides a network that has some power of 2 bits, as above a /24 provides 8 bits/ a /31, 1, /16, 16 etc.