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IPv4 Classful Network Exclusive Explanation

IPv4 Classful Network Exclusive Explanation

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A classful network is a network addressing architecture used in networks since 1981. RFC 790 and 791, released in 1981, describe how IPv4 network addresses were primarily allocated based on a classification system. The authors of IPv4 addresses set up three classes of network addresses: class A, B, and C for different network sizes.

Classful network addresses are defined with a specific format for the high-order bits (HOB). High-order bits (HOB) are the most significant bits in a 32-bit address. Classful network addresses remained in use until the introduction of Classless Inter-Domain Routing in 1993. The method divides the IP address space for IPv4 into five address classes based on the leading four address bits.

Class A Network

The high-order bit for Class A addresses is 0. Large organizations use this class. The address range starts from 0.0.0.0 to 127.255.255.255. The 0.0.0.0 address is reserved for default routing, and the 127.0.0.0 address is reserved for loopback testing.

Class B Network

The high-order bits for Class B Network addresses are 10. Medium-to-large organizations use this class. The range of Class B addresses is 128.0.0.0 to 191.255.255.255.

Class C Network

The High Order Bit for Class C Network addresses is 110. Small-to-medium organizations use this class. The address range starts from 192.0.0.0 to 223.255.255.255.

Class D Network

The High Order Bits for Class D addresses are 1110. This class is used for multicasting. Multicasting is a technique for finding a group of hosts that are part of a multicast group. The Range starts from 224.0.0.0 to 239.255.255.255.

Class E Network

The High Order Bits for Class E addresses are 1111. Class E address is also Reserved for experimental and future use. The range starts from 240.0.0.0 to 255.255.255.255. The address 255.255.255.255 is reserved for switch broadcasting.

High Order Bit (HOB)

The most significant bit (MSB), also known as the high–order bit, is the bit position in a binary number having the greatest value. It is the left-most bit due to the convention in positional notation of writing more significant digits further to the left. The table below illustrates the High-Order bit.

Classful Network Subnet Masks

The RFC 790 also define the default subnet mask for each network class.

Class A Address Subnet Mask

The classful network of class A used the first octet to identify the network portion of the address. The default subnet mask of Class A is 255.0.0.0. The first bit of Class A address from 0.0.0.0 to 127.0.0.0 is always 0; only 7 bits were left in the first octet. So, this made 2 to the 7th power or 128 networks. However, two networks, 0.0.0.0 and 127.0.0.0, are reserved for default route and loopback testing.

So, the actual number of networks for Class A is 126 (1-126) networks. The remaining 24 bits use the host in the host portion, so each class A address had the potential for over 16 million individual host addresses. The host portion must be 0, and the network portion must be 1s in the subnet mask. The figure below illustrates the Class A address default subnet mask.

Class B Address Subnet Mask

The Classful Class B networks used the first two octets to identify the network portion of the network address. The first two bits of the first octet, 10, define the network class, so 14 bits in the first two octets define the number of networks. The number of the network in a class B network is 2 the power of 14th, meaning 16,384 class B network addresses.

The 3rd and 4th octet is for the host, so each class B classful network address contains 16 bits in the host portion, meaning the number of hosts per subnetwork of Class B is 2, the power of 16th, the resulting host per classful network is 65536. The figure below illustrates the subnet mask of the Class B Address.

Class C Address Subnet Mask

The class “C” Classful networks used the first three octets to identify the network portion of the network address. The first three bits, 110, recognized the network class, and the remaining 21 bits for assigning networks for over 2 million class C networks. So, for each class C network, only 8 bits stay in the host portion. The 8 bits mean 254 possible host addresses in each Class C network. The figure below illustrates the Class C network subnet mask.

Classful Routing Protocol

The specific default subnet masks to each class made routing update messages smaller because the Classful routing protocol does not include the subnet mask information in their updates. So, the receiving router applies the default mask based on the value of the first octet or, more accurately, the first three bits of the address, which identifies the class.  Routing protocols, such as RIPv1, only need to transmit the network address of known routes.

It does not need the subnet mask in the routing update. The receiving router examines only the value of the first octet as the network address and determines the subnet mask or applies its ingress interface mask for subnetted routes.

Classful Network Addressing Waste

Classful network addresses waste a vast amount of address space. At the beginning of the Internet, organizations were assigned an entire classful network address from the A, B, or C class, resulting in IP address waste. So, allocating IP addresses in classful network addressing is very inefficient.

Class A addresses had 50% of the total IP addresses. However, it has only 126 networks, so it can be assigned to 126 organizations. The class A network uses 24 bits in the host portion, so we can calculate the IP address of each network, which is 224-2 = 16777214 addresses.

Classless addresses reduce this waste of gigantic IP addresses, but some companies and governmental organizations still have class A addresses. For example, General Electric has 3.0.0.0/8, Apple owns 17.0.0.0/8, and the U.S. Postal Service owns the 56.0.0.0/8 network.

Class B had 25% of the total address space. It has 16,384 networks, so 16384 organizations can get and use class B network addresses. A network uses 24 bits in the host portion, so we can calculate the IP address of each network, which is 216-2 = 65534 host addresses.

Class C had 12.5 % of the total address space. A network uses 8 bits in the host portion, so we can calculate the IP address of each network which is  28-2 = 54 host addresses. A small organization can get and use class C networks, but the total number of hosts is often limited in the class C networks. Classes D and E are used for multicasting and reserved addresses. It had 12.5 % of the address space. The pie chart illustrates the address space in different classes.

classful network addresses

The chart below illustrates the total network, host per network and maximum hosts in Class A, B, and C.

classful network addresses
Classless Subnetting Examples – Exclusive Details

Classless Subnetting Examples – Exclusive Details

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In the previous article, we have discussed the examples of classful subnetting which is too simple. we have borrowed host bits from the standard/8, /16, and /24 network prefixes. However, we can borrow bits from any host bit position using classless subnetting to create other masks. For example here, a /24 network commonly subnetted by longer prefix lengths by borrowing bits from the fourth octet. The administrator can assign network addresses to a smaller number of end devices using classless subnetting with the longest prefix and a smaller network. The figure below illustrates the /24 network further into smaller networks.

classless subnetting
  • The first columns illustrate the prefix length of each subnet after borrowing bits from the fourth octet.
  • The second columns illustrate the subnet mask for each subnetted network.
  • The third columns illustrate all the network, host, and borrowed bits in the subnet mask. Capital N shows network bits, Capital H shows host bits, and small n shows borrowed bits.
  • The fourth column illustrates the number of usable hosts per subnetted network.
  • The last column illustrates the number of available sub-networks after borrowing bits.

Classless Subnetting Example

The figure below illustrates the private network with /24 prefix. The network is 192.168.200.0. The first three octets are displayed in decimal, while the last octet is displayed in binary because we will get the borrowed bit from the fourth octet to create more sub-networks.

The subnet mask indicates that the prefix length is 24 bits. The first three octets are the network portion, and the last is the host portion, as shown in the figure above. With /24 prefixes (without subnetting), this network provides 254 usable host addresses supporting a single LAN.

The network would need subnetting if we required an additional LAN from the same IP network (192.168.200.0/24 ). The following are some questions/problems related to subnetting the same IP network.

The administrator required two sub-networks from network 192.168.200.0/24 network. So, first of all, we must have the answers to the following questions.
  1. Total IP addresses with /24 prefix?
  2. What are the total usable IP addresses with /24 prefixes?
  3. What is the Network Address?
  4. What is the Broadcast Address?

So, first of all, we will explain the above questions. We know that there are a total 32 bits in IP address /24 means that 24 bits are parts of the network portion and the remaining 32-24 = 8 bits are the parts of the host portion, and we know that:

subnetting

So we have required 2 sub-networks from the above network and all the above answers for both sub-networks. Remember that the fourth octet is displayed in binary because we will be borrowing bits from this octet to create more sub-networks.

The first question is how many bits we should be borrowing for two networks; the formula for the network is “2n = number of networks.”

If we put 21 = 2, we should borrow 1 bit from the host portion, as shown in the figure below. 1 bit is borrowed from the most significant bit (leftmost bit) in the host portion, extending the network portion to 25 bits or /25. The borrowed bit must be converted from 0 to 1 because the network bits are always 1’s, and the host bits are always 0’s.

The figures in borrowed bits should be different for each subnet. The two subnets result from varying the value of the borrowed bit, either 0 or 1. The figure below illustrates both sub-networks.

The octet in Net 0 is 00000000, and in Net one is 10000000. If we convert the fourth octet back to decimal we can see the resultant subnets that are 192.168.200.0/25 and 192.168.200.128/25.

The figure below illustrates both subnets with the resultant subnet mask. Notice how it uses a 1 in the borrowed bit position to indicate that this bit is now part of the network portion. The Figures also display the dotted-decimal representation of both subnet addresses and their ordinary subnet mask. The subnet mask for each subnet is 255.255.255.128 or /25 because it has one borrowed bit.

The figure below displays the necessary addresses for subnets 192.168.200.0/25 and 192.168.200.128/25.

  • Network addresses are 192.168.200.0/25 and 192.168.200.128/25, and both contain all 0 bits in the host portions.
  • Both networks’ first usable host addresses are 192.168.200.1 and 192.168.200.129; both contain all 0 bits, plus the right-most bit in the host portion is 1.
  • The last usable host addresses of both networks are 192.168.200.126 and 192.168.200.254. Both contain all 1 bits in the host portion except the right-most and last bit in the host portion, which is 0.
  • Broadcast addresses of both networks are 192.168.200.127 and 192.168.200.255, containing all 1s in the host portion of the IP address.

Example 2 – Required 4 sub-networks from the same Network 192.168.200.0/24.

Now we require 3 sub-networks from the same private network address 192.168.200.0/24. The first question raised is as:

How many bits are required to borrow from the host portion to the network portion for 4 sub-networks. ?

Borrowing a single bit provides 2 subnets, so borrow another host bit. Borrowing 2 bits results in (22 = 4) subnets. So, we should borrow 2 bits from the host to the network portion, as shown in the figure below. The subnet mask or network prefix of the subnetted network is changed to /26 or 255.255.255.192.

The value XX illustrates that the bit should be changed for each network. We have borrowed 2 bits from the host portion. So, in the first network, or Subnet-0, the value for XX = 00, Subnet-1 the value for XX=01, Subnet-2 the value of XX=10, and Subnet-3 the value of XX=11.

We can determine the number of hosts per network by looking at the last octet in the figure above. After borrowing 2 bits from the subnet from the fourth octet, 6 host bits remain. Apply the host calculation formula (2h-2= Usable host) to reveal that each subnet can support usable 62 host addresses. The figure below displays the significant addresses of each subnet.