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This question is fairly basic to those in this community...

  • Is 192.168.X.X/24 (255.255.255.0) a class C private address?

  • If so, can someone explain how is it possible to have a subnet mask greater than 255.255.0.0? I thought 255.255.0.0 was the default/natural mask of a class C private?

I have some understanding that CIDR is different from classful. In this example, however, my professor went on to subnet the networks, separating them by 192.168.10.0 and 192.168.20.0 which made no sense to me considering the original question was written with /24.

  • If it is of the private class C range, why would the 3rd octet be usable?

While I know this question is outdated, I'm studying for Net+/CCNA so I need to know, unfortunately.

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Is 192.168.X.X/24 (255.255.255.0) a class C private address?

You got it and you usually see it like this because of it being the default on so many home routers.

If so, can someone explain how is it possible to have a subnet mask greater than 255.255.0.0? I thought 255.255.0.0 was the default/natural mask of a class C private?

Greater as in 255.0.0.0 or 255.255.255.0? It's possible because you have 4 octet's in a Ip4 address. Which gives you 4 octets to mask.

In this example, however, my professor went on to subnet the networks, separating them by 192.168.10.0 and 192.168.20.0 which made no sense to me considering the original question was written with /24.

Ahh this is where you got confused. A subnet mask will tell you what part of an ip address is the network part. So if you have 192.168.XXX.XXX /16 or 255.255.0.0 we can see that only the first 2 octets are part of the network address and the last 2 octets are part of the host address. so any Ip's that start with 192.168 are now part of the same network and anything in the third octet is only used to identify a host.

So, if you take 192.168.10.xxx and 192.168.20.xxx /16 they are in the same network (192.168.xxx.xxx) if you change the mask to /24 (255.255.255.0) the first 3 octet's are now Network's and have to match. So 192.168.10.xxx and 192.168.20.xxx 10 and 20 don't match, they are different networks, and only the last octet is used to identify host on the network (255.255.255.0 255 is for networks, 0 is for host), and this gets much more advanced so get a good understanding of it now while you can. For instance you can use a ACL to route traffic to one host only. say 192.168.10.123 all you have to do is set the netmask to 255.255.255.255 and it will make sure it only goes to the host that matchs ever number in every octet.

If it is of the private class C range, why would the 3rd octet be usable?

Now you know it's because of the netmask /24 =)

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Levi and Pieter both have the right idea. The Internet Engineering Task Force (IETF) discusses IPv4 Private Address Space here: https://tools.ietf.org/html/rfc1918

"The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of the IP address space for private internets:

 10.0.0.0        -   10.255.255.255  (10/8 prefix)
 172.16.0.0      -   172.31.255.255  (172.16/12 prefix)
 192.168.0.0     -   192.168.255.255 (192.168/16 prefix)

We will refer to the first block as "24-bit block", the second as "20-bit block", and to the third as "16-bit" block. Note that (in pre-CIDR notation) the first block is nothing but a single class A network number, while the second block is a set of 16 contiguous class B network numbers, and third block is a set of 256 contiguous class C network numbers."

So, with a network of 192.168.0.0/24, you are specifying a different network than 192.168.10.0/24 or 192.168.20.0/24. If the network is 192.168.0.0/16 as rfc1918 specifies, all three of these addresses would be on the same network.

0

If it's a /24, it means all of the first three octets are used up with 1s, 255.255.255.0 is the subnet mask that matches /24 because all 8 bit in the first three octets are used, 8 x 3 = 24. With the first octet in this network being 192 this is clearly class C. Therefore, there can be no manipulation of the third octet within any particular network. What he might be doing is setting up more than one class C network, each one separate and distinct, needing a router in order to communicate between networks. It seems to me he is just choosing .10 increments to make room for growth. Access my subnetting videos on youtube and learn subnetting the easy way.

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    An address's class has nothing to do with its netmask used other than the default mask for the class. Address classes are defined by the values of the first bits in the address. – Ron Maupin Oct 20 '15 at 15:44
  • A class C is defined by the first octet. It's 192 in this case, meaning it is a class C, no need to go further with that issue.. The /24 tells us it's a class C that has not been subnetted. If it had been subnetted the forth octet in the subnet mask would be something other than 0 or 255. And the /CIDR would be larger. Because class C start at /24 and go up. Only a supernet class C has a lower /CIDR than /24. – pokerfacetodd Oct 20 '15 at 15:58
  • A Class C address is defined, not by the first octet, but by the first three bits being 110. The 24-bit mask of a Class C address is nothing more than the default mask for that class. By definition, a Class C address could have a mask length as small as /3 or as large as /32. The class of an address is based purely on the first bits of the address. – Ron Maupin Oct 20 '15 at 16:04
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    You should the read RFC that defines the classes. Class C is defined by its first three bits being 110. That means the smallest mask length that preserves the first tree bits is /3. That /24 is the default mask for Class C has no relationship to what constitutes a Class C address. – Ron Maupin Oct 20 '15 at 16:37
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    The RFC says that Class C addresses start with 110 which would mean that the scope of Class C addresses is 192.0.0.0 to 223.225.225.225. The default mask for a Class C address is /24. You could have 192.0.0.0/3 as a network defined on a router without violating any rules. It encompass the entire Class C address range, and all the addresses for that network would be available for host addresses except 192.0.0.0 and 223.255.255.255, or you could assign a loopback with 192.0.0.0/32 (or any individual Class C address). The mask size doesn't matter, but the default for Class C is /24. – Ron Maupin Oct 20 '15 at 17:34
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See questions/19840 for CIDR discussion.

Is 192.168.X.X/24 (255.255.255.0) a class C private address?

  • Yes that is private address space, and Yes a mask of /24 is called a class C mask.

explain how is it possible to have a subnet mask greater than 255.255.0.0?

Classfull subnetting is not used any more, although a lot of people still refer to a 255.255.255.0 mask as a ClassC subnet.

With CIDR taking any network with a mask /x you can subdivide it with a mask /y as long as y>x up to /32 the host mask.

Taking your example:

192.168.X.X/24 can not be sub divided into more /24 networks.

192.168.X.X/24 does indicate that you are talking about one of 256 possible networks, 192.168.0-255.x/24.

The network 192.168.0.0/16 can be sub-netted into 192.168.10.0/24 and 192.168.20.0/24

  • Ah that makes sense. So really /16 is just the default mask and anything greater than that can theoretically be used based on how many subnets/hosts per subnet you will need? I was under the impression that anything equal to or greater than /24 would result in only using the last octet. – oxy Jul 21 '15 at 8:17
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    /24 does not necessarily mean Class C. Network classes are defined by the leading bits of an IP address. 0 == Class A, 10 == Class B, 110 == Class C, 1110 == Class D, 1111 == Class E. – Jens Link Jul 21 '15 at 9:43
  • @Jens Link to be Class C in Classful subnetting, also implies 24bits for the network portion thus /24, the reason poeple refer to 255.255.255.0 as a Class C mask. – Pieter Jul 21 '15 at 20:28
  • @Pieter, subnetting a Class C network would imply a mask greater than /24. The /24 is the mask length of a Class C network, while a subnet of a Class C network would have a larger mask length. Subnetting was invented as a way around classful networks. If you mean classful routing, that is, for all practical purposes, dead since RIPv1 and IGRP are history. There is no real reason to refer to, or use, classful networks any longer. Personally, I think the whole classful thing confuses beginners who should just learn to mask addresses, period. – Ron Maupin Jul 22 '15 at 3:09
  • @RonMaupin I understood what you all mean. I think as Jens says, learning the leading bits to classify them while leaving out the default subnet mask of each it would be much easier. It's much easier understanding the typical CIDR of each class, rather than being under the implication that once CIDR is written, that is the ONLY possible CIDR notation. When my teacher gave me /24, I didn't know I could go back to /16 when creating the different networks if that makes any sense. – oxy Jul 22 '15 at 3:56

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