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The classification of addressing spaces in computer networks is carried out by means of so-called network segments or subnets. The size of the individual networks depends on their classification. Class C IP is probably the most widespread size in corporate networks.
A Class C IP is an IP address in a Class C segment. The network classes have their origin in the beginnings of the Internet. By the introduction of special techniques an efficient division into subnets could be made possible. From a certain size a flexible handling is necessary when dividing the network segments in order to allow the individual computers of a network to communicate with each other. The forwarding of data packets from the sender to the receiver across subnet boundaries is handled by a so-called routing instance. In order to understand the complicated interrelationships behind these terms, the individual relevant points are explained below.
Computers and other end devices such as printers or smartphones have a MAC address for identification in the network. Since these hardware addresses can only be reached in the same subnet, an instance in a higher layer is required, which can be reached across network boundaries.
This task is performed by the IP address. It is a logical assignment to the MAC address and consists of a 32-digit binary number of ones and zeros. For better readability, the 32 digits with dots have been divided into four eight-digit octets. In a further step, these address parts of the size of one byte were converted into decimal values. This is how the current representation of an IP address of version 4 (IPV4), which prevails on the Internet as well as in most corporate networks, was created. For example, the binary notation "11000000101010000111101110000100" becomes the readable Class C IP 192.168.10.142.
The origin of the classification of IP addresses lay in the network classes, which were a rigid division into network area and computer area. The first 8 bits - i.e. the first octet - were always provided for the network segments. The other 24 bits - i.e. the second to fourth octet - were for the end device itself. The entire available address space was initially divided into three classes A, B and C via these network classes. Later, two further classes, D and E, were defined for special purposes. Class A included all addresses from 0.0.0.0 to 127.255.255.255. Class B included the address ranges from 22.214.171.124 to 126.96.36.199. Class C extended from 192.0.0.0 to 188.8.131.52. The netmasks or subnetmasks were decisive here.
Class A had the netmask 255.0.0.0.0, class B had the netmask 255.255.0.0 and class C had a netmask with the value 255.255.255.0 for the identification of net and computer areas. The netmask was not yet important for the size or division of a net segment into a net class. The value of the first octet or byte was decisive. Since the individual nets had already been segmented into subranges, the netmask had a meaning for this original form of subnetting.
With the introduction of Class Inter-Domain Routing (CIDR), the importance of the netmask changed. They became the most important factor for data traffic with IP addresses. With it, the four octets were and are divided into the respective network and computer areas. Analogous to the IP address, the netmask also has four bytes. This means that the size of a network can no longer be derived from the IP address alone. A netmask is always mandatory. The "0" always indicates the computer part.
In the classical variant, there was only the 255 for the network shares. Consequently, a Class C IP had a netmask of 255.255.255.0, with the first three bytes representing the network. Only the last byte defines the range for the end devices in a Class C subnet. The network classes themselves only have a minor significance due to the net masks. However, they form the basis for calculating the subnets using CIDR and VLSM. Therefore, the term Class C IP is still relevant today.
Class Inter-Domain Routing (CIDR) was introduced in 1993. The fixed assignment of an IP address was replaced by the specification of a netmask. Each IP address can only be routed via a corresponding netmask. For the first time, so-called suffixes were used for notation with CIDR, which indicate the number of 1-bits in a netmask. The abbreviated form 192.168.10.142/24 corresponds to the representation of the IP address 192.168.10.142 (netmask 255.255.255.0). With CIDR, the subnet masks with variable lengths were also introduced into the network world. The technology that made this possible is called Variable Length Subnet Mask (VLSM) and changed everything.
With VLSM the subnetting was extended. Thanks to VLSM, the division of larger network areas could and can be carried out down to the smallest possible subnetworks. A larger subnet is divided into several smaller subnets, which are assigned a corresponding subnet mask. For example, the IP address 192.168.92.140 from the higher-level Class C IP subnet is found in the subnet with the address range 192.168.92.129 to 192.168.92.142 using the subnet mask 255.255.255.240.
Subnetting, by which the subdivision of a network into several subnets is meant, has become a more complicated matter through the use of VLSM, which can only be completely mastered by die-hard professionals. For the daily work of a network administrator, there are special applications that automatically calculate the subnets and the associated address ranges. Thanks to the subnet masks, the smallest possible network consists of only six addresses. Conversely, a Class C IP can also belong to a subnet of a larger Class B or Class A network. A Class A IP such as 10.158.18.22, for example, becomes a Class C IP within an A class address space by means of the 255.255.255.0 subnet mask.
Routing plays a crucial role both in larger corporate networks and on the Internet. The entire Internet consists to a large extent of routers, which form the third large unit of the World Wide Web alongside the web servers and client computers. The routers do not pay attention to the destination address of the computer. They concentrate exclusively on the subnets in which the sender and recipient of a message are located.
By means of one or more routing protocols, all relevant routers in a network or on the Internet have a table with the ways in which the individual subnets can be reached. For example, the data packet of a Class C IP is sent across various network boundaries to the target computer with a completely different Class C IP.
It is technically impossible to assign unique IP addresses to all networks and computers on the Internet and in corporate and home networks worldwide. The maximum number of possible IPs is not sufficient. As a result, corporate networks and other facilities that were not directly connected to the Internet were provided with private address ranges by the IANA ("Internet Assigned Numbers Authority"). This authority, which assigns or monitors all IP addresses and other important procedures on the Internet, assigned the private address ranges which never appear on the Internet and are routed.
If other addresses are used in a company network, they cannot be routed directly to the Internet because they are public address ranges. Address conflicts would arise and data transfer would be rejected. Within the Internet, all IP addresses can only be present once.
Network Address Translation (NAT) is the solution to this problem. A company firewall has a public IP and uses it to replace the IPs of the private computers in the company network in order to route the data packets on the Internet. Conversely, the address headers of all incoming data are provided with the corresponding private destination address by the firewall, since the Internet addresses cannot be forwarded in the private company networks.