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Public IPv4 addresses

We spoke about IPv4 addresses in a few discussions previously. In this section, we'll go even further and discuss the actual grouping of the addresses.

There are two main IPv4 address spaces—the public address space and the private address space. The primary difference between both address spaces is that the public IPv4 addresses are routable on the internet, which means that any device that requires communication to other devices on the internet will need to be assigned a public IPv4 address on its interface, which is connected to the internet.

The public address space is divided into five classes:

Class D addresses are used for multicast traffic. These addresses are not assignable. Class E addresses are reserved for experimental usage and are not assignable.

On the internet, classes A, B, and C are commonly used on devices that are directly connected to the internet, such as layer 3 switches, routers, firewalls, servers, and any other network-related device. As mentioned earlier, there are approximately four billion public IPv4 addresses. However, in a lot of organizations and homes, only one public IPv4 address is assigned to the router or modem's publicly facing interface. The following diagram shows how a public IP address is seen by internet users:

So, what about the devices that require internet access from within the organization or home? There may be a few devices to hundreds or even thousands of devices that require an internet connection and an IP address to communication to the internet from within a company. If ISPs give their customers a single public IPv4 address on their modem or router, how can this single public IPv4 address serve more than one device from within the organization or home?

The internet gateway or router is usually configured with Network Addresses Translation (NAT), which is the method of mapping either a group of IP addresses or a single IP address on the internet-facing interface to the local area network (LAN). For any devices that are behind the internet gateway that want to communicate with another device on the internet, NAT will translate the sender's source IP address to the public IPv4 address. Therefore, all of the devices on the internet will see the public IPv4 address and not the sender's actual IP address.