Why Most IP Addresses Start with 192.168 – A Deep Dive into IPv4, Private/Public IP, CIDR and NAT

To start, the author shares a humorous story about a colleague, Xiao Zhang, who believes he is already living with his crush because they share the same local IP range (192.168.xx.xx), illustrating how common that prefix is.

What is an IP address? An IP address works like a mailing address for data packets on the Internet; it tells routers where to deliver each packet, just as a street address tells a courier where to deliver a package.

Because the current Internet mainly uses IPv4, the article focuses on IPv4 addressing.

IPv4 addresses are 32‑bit numbers, usually written as four decimal bytes separated by dots (e.g., 192.168.31.170). Each byte is 8 bits, giving a total of 2^32 ≈ 4.2 billion possible addresses.

On a Unix‑like system you can see your local IP with the ifconfig command; the inet field shows the address, such as 192.168.31.170 :

$ ifconfig
en0: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
    ether 88:36:3d:33:a0:15
    inet6 fe70::1009:aabf:ecc6:2d10%en0 prefixlen 64 secured scopeid 0x6
    inet 192.168.31.170 netmask 0xffffff00 broadcast 192.168.31.255
    nd6 options=201<PERFORMNUD,DAD>
    media: autoselect
    status: active

In plain terms, an IP address is a unique identifier that allows a device to be located on a network.

Because an IPv4 address consists of 32 bits, it can represent up to 4.2 billion hosts. In 2021 the world already had about 7.8 billion IPv4‑compatible devices, showing that the address space is insufficient for the growing number of devices.

This shortage led to the development of IPv6, which uses a much larger address space (128 bits) and looks very different from IPv4.

Historically, IPv4 addresses were divided into three "classful" networks:

Class A: first bit 0 (e.g., 10.0.0.0 – 10.255.255.255)

Class B: first bits 10 (e.g., 172.16.0.0 – 172.31.255.255)

Class C: first bits 110 (e.g., 192.168.0.0 – 192.168.255.255)

Each class splits the 32‑bit address into a network part and a host part, similar to a parking lot (network) and individual parking spaces (hosts). Class A provides few networks but many hosts, while Class C provides many networks but few hosts.

Because the classful system is inflexible, modern networks use CIDR (Classless Inter‑Domain Routing), which specifies the length of the network prefix with a slash, e.g., 172.20.61.69/20 . Here, the first 20 bits are the network ID and the remaining 12 bits identify hosts, allowing up to 4096 hosts in that subnet.

Private IP ranges (defined in RFC 1918) are not routable on the public Internet and are used inside local networks:

* A‑class private: 10.0.0.0 – 10.255.255.255
* B‑class private: 172.16.0.0 – 172.31.255.255
* C‑class private: 192.168.0.0 – 192.168.255.255

Most home networks use the C‑class private range (192.168.xx.xx) because it provides enough addresses for typical households while keeping the address space simple.

In a typical setup, many devices inside a LAN share a single public IP address provided by the ISP; the router performs NAT (Network Address Translation) to map internal private addresses to the external public address.

To discover your public IP, you can simply search "my IP address" on a search engine (e.g., Baidu) or use an online service. The article shows an example where a cloud server has both a private IP (e.g., 172.21.56.59) and a public IP (e.g., 46.101.121.11). The private IP cannot be pinged from the Internet, while the public IP can.

Cloud providers obtain blocks of public IPs from regional Internet registries and lease them to customers as needed. Multiple virtual machines can share a single public IP using NAT, which mitigates the IPv4 exhaustion problem.

Summary

IP addresses are unique numeric identifiers that enable routing of data packets across the Internet.

IPv4 provides about 4.2 billion addresses; the historic classful system (A/B/C) divided them for management, but CIDR is now the standard.

Private IP ranges (10/8, 172.16/12, 192.168/16) are used inside LANs; public IPs are used on the wider Internet.

Home networks typically use the 192.168.x.x private range, which explains why many personal devices share that prefix.

You can view your local IP with ifconfig and your public IP via a web search or online service.

Finally, the article poses a question: where does the translation between private and public IPs (NAT) actually occur? The answer is that it happens on the router or gateway that connects the LAN to the ISP.