Private IPv4 addressing were introduced as a solution to a major issue: the exhaustion of public IPv4 addresses.
Back in 1981, the IPv4 Public Address space was designed with about 4.3 billion unique addresses. At that time, the creators of the Internet did not anticipate how massive adoption would become.Over the years, public IPv4 addresses managed by IANA and the Regional Internet Registries (RIRs) allocated worldwide. As more networks and devices were deployed, the number of available public addresses kept shrinking.
Figure 1 – IPv4 Public Address Exhaustion Curve
By the early 1990s, engineers realized that the public IPv4 space would not be sufficient.
To slow down this depletion, several measures were introduced:
1993 – CIDR (Classless Inter-Domain Routing): allowed more flexible allocation of addresses.
1996 – RFC 1918 (Private IPv4 Addresses): reserved specific ranges for internal networks to avoid wasting public addresses.
Despite these measures, the IPv4 public pool eventually ran out and was officially exhausted in 2019.
Answer the question below
Private IPv4 addresses were formally defined in RFC 1918 (1996) by the IETF (Internet Engineering Task Force).
They are special IPv4 ranges reserved exclusively for internal networks such as homes, enterprises, and campuses.
The introduction of private addressing changed the way internal networks were designed and organized. Instead of consuming valuable public IP space, organizations could freely use private ranges for their internal devices. This helped preserve the limited pool of public IPv4 addresses.
Figure 2 – Comparison between Public IPv4 and Private IPv4 Addresses
Advantages of Private IPv4 Addresses
Cost savings – no need to purchase or request public IPs for internal devices.
Security by isolation – private IPs are not reachable from the Internet.
Flexibility – the same ranges can be reused by different organizations without conflict.
Answer the question below
Which RFC defines private IPv4 addresses?
The RFC 1918 standard defines three reserved ranges for private use:
Figure 3 – RFC 1918 Reserved for Private IPv4 Addressing
These ranges were chosen for convenience because they aligned with the familiar classful IPv4 boundaries.
10.0.0.0/8 → was taken from Class A.
172.16.0.0/12 → is a subset of the Class B space.
192.168.0.0/16 → is a subset of the Class C space.
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What is the Class A private range?
In practice, enterprises build their internal networks using the RFC 1918 ranges combined with CIDR subnetting.
Since private addresses are not routable on the Internet, they can be reused by multiple organizations without conflict.
Figure 4 – Multiple companies reusing the same private IPv4 subnet
For example, Company A, Company B, and Company C could all use 192.168.1.0/24 inside their networks, and it would work perfectly fine as long as they are not directly interconnected.
Private IPs are typically used for devices that don’t need permanent public addresses, such as:
User devices (PCs, smartphones, tablets),
Printers,
Switches and Routers within internal networks.
How Do Private Hosts Reach the Internet?
At this point, an important question arises:
“If private IP addresses are not unique and not routable on the Internet, how can these devices connect outside their local network?”
The answer is that on the global Internet, only public IPv4 addresses are routable.
When a device with a private IP needs to communicate with the Internet, it must rely on NAT (Network Address Translation).
Figure 5 – NAT translating private IPv4 addresses to public ones
Private IP addresses are converted into public IP addresses before reaching the Internet.
For example, when a host sends packets with a private source address, the router at the edge of the network (often the ISP router) replaces that private address with a public one, then forwards the packets to the Internet.Answer the question below
What allows private IPs to access the Internet?
Private IPv4 addressing, combined with CIDR, is widely used inside corporate networks to reduce the need for public IP addresses. Since private addresses are free and can be reused in any organization, they provide flexibility and efficiency in network design.
However, private IPs are not routable on the Internet, so whenever communication with the outside world is needed, NAT must be implemented to translate private addresses into public ones.
For your CCNA exam, make sure you remember the three private IPv4 ranges:
10.0.0.0/8 → Class A.
172.16.0.0/12 → Class B
192.168.0.0/16 → Class C
Answer the question below