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classful addressing

IPv4 Classful and Classless Addressing

IPv4 Classful and Classless Addressing

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Classful and Classless addressing are both terms that refer to a viewpoint on the structure of a subnetted IP address.  Classless addressing has a two-part view, and classful addressing has a three-part view.  With classful addressing, the address always has an /8, /16, or /24 bit network prefix, based on the Class A, B, and C addressing rules.  The end of both address types has a host part that uniquely identifies each host inside a network or sub-network.  The classful address has a third part between the network and the host part, namely the subnet part of the address containing borrowed bits.

Classful Addressing

Each router interface connected to a network requires an IP address and subnet mask. The IP address uniquely identifies the router or device in the network, and the subnet mask identifies the network broadcast. The prefix length and the subnet mask are different ways of determining the network portion of the IP address. For IPv4 subnetting, we borrow host bits and use them as network bits to create supplementary networks called sub-networks. We can subnet a network, either a classful or a classless subnet.

Borrowing more host bits can define more sub-networks. We already learned that an IPv4 address has 4 octets, and the octet boundaries are predefined:/8, /16, and /24. Network subnetting at octet boundaries is too easy. The table below identifies the prefix lengths and subnet masks equivalent to prefixes. The table also identifies the network and host bits, including the number of hosts each subnet can connect. The longer prefix lengths mean fewer hosts per network.

Classless

Classful Subnetting

Subnetting on the octet boundary is easy and is also known as classful subnetting. To understand the subnetting on the octet boundaries, examine the following example. Suppose you have an IP network 120.0.0.0/8 in a single broadcast domain. The number of hosts in the network is 16,777,214. This is a large network with an extensive broadcast, causing slow network performance. So, this is not perfect.

You can subnet the network 120.0.0.0/8 address at the octet boundary of /16, as shown in the Figure below. Subnetting with /16 can provide 256 sub-networks (i.e., 120.0.0.0/16 – 120.255.0.0/16). Each subnet contains 65,534 hosts. The first two octets with the/16 prefix length identify the network portion of the address, and the last two octets are for the host portion of IP addresses. The figure below illustrates the subnetting of /8 network into /16 network.

As shown in the Figure below, you can also subnet the /8 prefix network into the /24 octet boundary. A/24 subnetting can provide 65536 sub-networks (i.e., 120.0.0.0/24 – 120.255.255.0/24), with each subnet containing 256 hosts. The first three octets identify the network portion of the address, while the last octets are for the host portion of IP addresses while using the/24 prefix.

Classless Subnetting

Classful subnetting uses the default Class A, B or C networks, including the default mask for its classes (A, B, C). The default subnet mask for Classes are:

Class-A: 0 – 127 with a mask of 255.0.0.0 or /8

Class-B: 128 – 191 with a mask of 255.255.0.0 or /16

Class-C: 192 – 223 with a mask of 255.255.255.0 or /24

Routers configured with classful ip addresses do not include subnet mask information with routing updates. The router assumes its subnet mask or defaults to the classful subnet mask.

However, classless subnetting allows the use of Variable-Length Subnet Masks (VLSM). The classless IP address uses a CUSTOM subnet mask. Routers running a classless IP address, for example, include subnet mask information with their routing updates.

120.210.0.0/17

20.6.150.0/28

172.31.16.0/21

172.16.16.0/20

Subnetting Formulas

To calculate the number of subnets produced by the bits borrowed from the host portion, use formula (1), and for the number of hosts per network, use formula (2).

The “n” is the number of borrowed bits from the host portion. The first formula will produce the number of possible networks. The “h” is the number of bits remaining in the host portion. The second formula will produce the number of usable host addresses for each sub-network. Two addresses cannot be assigned to any host: the network address(the first address of the subnet) and the broadcast address(the last address of the subnet), so we must subtract 2 for usable host addresses.

Classful and Classless IP Addressing – Exclusive Details

Classful and Classless IP Addressing – Exclusive Details

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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.