I'm new to networking and I'm doing my best to understand how things work. I do have a problem in understanding IP scheme idea (even though obsolete), more specifically, their structure, distribution by organizations and usage.

To my understanding, we have 2^32 addresses that can be used through the protocol IP and the distribution of these addresses is handled by certain organizations (IANA, ICANN, RIRs...).

1. Understanding scheme and distribution of IP addresses

If an organization wanted to join the internet, it had to request an unique IP address. At the beginning it was considered that the 2^32 IP addresses will not be used by people but organizations. Due to this, they organized the addresses in specific categories, distributing to organization IP address(es) from a certain category based on the size of the organization.

IANA had to choose a specific pattern in the address to easily identify the magnitude of the IP address hosts and networks. Therefore, they said let's use the first bit of the first octet as the identifier of the IP category/class to understand its magnitude. This resulted in the range of each IP category:

  1. CLASS A: 255.x.x.x (1.x.x.x - 127.x.x.x)
    • Through this category, we can distribute up to 2^6 IP addresses so each organization will be able to use 2^24 addresses in it's topology.
  2. CLASS B: 255.255.x.x (128.0.x.x - 191.255.x.x)
    • Through this category, we can distribute up to 2^14 IP addresses so each organization will able to use 2^16 addresses in it's topology.
  3. CLASS C: 255.255.255.x (191.0.0.x - 223.255.255.x)
    • Through this category, we can distribute up to 2^21 IP addresses so each organization will able to use 2^8 addresses in it's topology.

!Some address space from each class has been reserved for private use only in order to not conflict with the public addresses usage.

To my understanding, if an entity needs an IP address, it will simply be given an IP address from a specific class. For example: Class C 191.1.1.x, meaning that it can use this IP address and all its range to

Is this correct ?

2. Usage of the IP address in networking / CIDR

A device (layer 3) was previously able to route a packet to destination based on the network info from the first byte of the IP address. This was the initial paradigm which now is NO longer used as we moved to CIDR scheme, limiting an IP network only by the first byte wasn't scalable.

As of now, we have requested an IP address and we received network 11.1.1.x for the use in the organization.

  1. As classes are no longer used (hence checking first byte), who decides/dictates what's the subnet of this IP that we received ?
  2. If organization has IP 11.1.1.x , what does it stop it from subnetting it in any shape and mode and be public accessible in internet ?
  3. In the modern intern, do most of the private networks use the exact RFC 1918 private IP ranges ?
  4. In the modern internet, if an organization receives the 11.1.1.x address, upon deciding (1) the class, does the remaining address space is normally used to configure layer 3 devices so it can take advantage of using multiple nodes with public IP addresses behind different LANs ?

@EDIT: I'm not planning on using IP classes but to understand how it began and how it works in the modern times. Thanks in advance for your help

  • Classful networking died in 1993 when CIDR was introduced. Please don't ever use classes in real life.
    – Zac67
    May 19, 2020 at 11:56
  • Network classes are dead (please let them rest in peace), killed in 1993 (before the commercial Internet) by RFCs 1517, 1518, and 1519, which defined CIDR (Classless Inter-Domain Routing). Modern networking does not use network classes.
    – Ron Maupin
    May 19, 2020 at 13:42

1 Answer 1


Here's the #1 thing to understand about IP address classes:

Classfull addressing is OBSOLETE, and has been since before you were born.
You can learn about it for historical reasons for the same reason you'd learn about how to make a house out of mud, but it has no bearing on modern networking.

As classes are no longer used (hence checking first byte), who decides/dictates what's the subnet of this IP that we received ?

The entity that assigned you that address. It could be someone in your organization, or an Internet authority, such as www.arin.net or www.ripe.net.

This question and answer may help you.

  • I understand, however where I wanted to go with the discussion was to understand how are now IP addresses assigned by authorities and who decides upon the configuration of an IP address, more specifically, If authority gives me an IP address, who decides what's the mask for it ? Also, is this an complete ip like or they give you the entire range like ?
    – Floji
    May 19, 2020 at 12:05
  • Updated my answer.
    – Ron Trunk
    May 19, 2020 at 12:09
  • Thanks @Ron. So if I understand correctly, the public IP address comes together with a mask and so you can't do anything you want with the address. Additionally, if an authority gives me the public address /24, is it possible that someone else to have but with /16 ?
    – Floji
    May 19, 2020 at 12:19
  • No. Duplicate addresses are not allowed on any network (except for a very specific use case, which you can ignore for the time being). IP addresses are assigned to hosts. Subnet masks define networks.
    – Ron Trunk
    May 19, 2020 at 12:33
  • Thanks @Ron. One last question, when an entity requests an IP, does it receive the whole block after the network portion ? For example, /24, the "1.1.1" is the network that the company will use whereas the remaining byte will be used as it wishes. Naturally, it should receive the whole block..but just to make sure I'm on the right track.
    – Floji
    May 19, 2020 at 13:22

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