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classful network address ranges

IPv4 Classful Network Exclusive Explanation

IPv4 Classful Network Exclusive Explanation

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
Classful and Classless IP Addressing

Classful and Classless IP Addressing

Classful Addressing

The IETF published the first major addressing scheme in September 1981 in RFC 790. The IP addressing scheme was 32 bits long and had three classes, A, B, and C, corresponding to 8-bit, 16-bit, and 24-bit prefixes. No other prefix lengths were allowed then, and there was no concept of nesting a group of 24-bit prefixes, such as within a 16-bit prefix.

Class D and E addresses were also defined, but neither of these two address classes was normally used. Class D addresses are reserved for multicasting, and Class E addresses are reserved for experimental and future use. The easiest way to distinguish between different address classes is to use the first decimal number in the IP address. Classful networks use the classful subnet mask according to the leading bits in the first block of the IP address. The figure below illustrates the key information of the Classful address scheme.

classful

Class A (0.0.0.0 to 127.255.255.255)

The default subnet mask for this class is 255.0.0.0 or /8. This class supports an extremely large network with more than 16 million hosts. The first octet’s high-order bits of Class A addresses are zero, so the remaining 7 bits create 128 possible Class A networks. 0.0.0.0 is used for the default router, and the 127.0.0.0 network is reserved for local loop testing. So, the remaining network is from 1 – 126 total 126 networks.

Class B (128.0.0.0 – 191.255.255.255)

The default subnet mask for the class b network is 255.255.0.0 or /16. Class b network support supports large networks with up to 65,000 host addresses. The high-order bits for the class b network are 10 in the first octet, and the remaining bits of the first 2 octets create over 16,000 networks. The network 169.254.0.0 is a special network for link-local addresses, also known as Automatic Private IP Addressing (APIPA).

Class C (192.0.0.0  – 223.255.255.255)

The default subnet mask for a Class C network is 255.255.255.0 or /24. Class C supports small networks with a maximum of 254 hosts. The first three bits of the octet indicate the high-order bit of the class. The remaining bits of the first three octets indicate the network, and the fourth indicates host addresses in this class. The high-order bit is 110. A Class C address has over 2 million possible networks.

Class D (224.0.0.0  – 239.255.255.255)

The first four bits of the first octet in Class D IP addresses are high-order bits (HOB); the first four bits are 1110. The range of Class D addresses starts from 224.0.0.0 to 239.255.255.255. Class D is reserved for multicasting. In multicast communication, data is destined for multiple hosts, not for a particular host. The class has no subnet defined.

Class E (240.0.0.0  – 255.255.255.254)

The first five bits of the first octet are reserved HOB for Class E address. The HOB for Class E is 11111. The address range is 240.0.0.0 to 255.255.255.254. This class is reserved for experimental purposes only, such as R&D and study. Class E is also not equipped with a subnet mask like Class D.

Public IP Addresses

A public IP address range is defined for network devices, hosts, and servers like web servers and email servers to allow direct access to the Internet. Any server device using public IP addresses directly accessible from the Internet. A public IP address is globally unique and can only be assigned once to any device worldwide. Every device accessing the internet is using a unique IP address. Public IP addresses are also required for any publicly accessible network hardware, such as servers hosting websites. Public addresses are globally routed between different ISPs and routers. However, some addresses are not routable on the Internet. These addresses are called private addresses.

Private IP addresses

Private IPv4 addresses were introduced in 1990 because of reduced IPv4 addresses. The Private addresses are not unique and can be used repeatedly for internal networks. The computers at home, tablets, smartphones, network printers, and the computers within organizations are generally assigned private IP addresses. A computer with a private IP address can see and access the local network through its private IP address.

The computer and devices with a private IP address cannot directly access and communicate via the private IP address; however, using the router’s public IP addresses, the devices outside a private network can communicate. The NAT allows direct access to a local device assigned a private IP address. The range of private IP addresses is defined for all three classes.

10.0.0.0 /8 or 10.0.0.0 to 10.255.255.255

172.16.0.0 /12 or 172.16.0.0 to 172.31.255.255

192.168.0.0 /16 or 192.168.0.0 to 192.168.255.255

Classless Addressing

Classful addressing divides an IP address into the Network and Host portions along octet boundaries. It uses a fixed subnet mask, which is /8, /16 and /24, but classless addresses use a variable number of bits for the network and host portions of the address. The subnet mask is not fixed for a classless addressing system.

The classful addressing system assigned 50% of IPv4 addresses to Class A networks, 25% of IPv4 addresses to Class B, 12.5% of IPv4 addresses to Class C, and the remaining 12.5 % Shared to both Class D and E. The classful addressing plan wastes the most IP addresses, decreasing the availability of IPv4 addresses. For example, an organization with a network with more than 254 hosts would need a class B network with more than 65,000 addresses, wasting 64,700 IP addresses.

IETF introduced classless addressing to overcome the waste of IP addresses in 1993. There is no IP address class in a classless addressing system, but the addresses are still granted in blocks. In a classless addressing system, when an organization or individuals need connectivity to the Internet, it also grants a block or range of addresses according to the needs of the organization and individuals.  For example, an individual requires only two addresses, and an organization is given thousands of addresses based on the number of its requirements.

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