03 — IP CLASSES & PRIVATE NETWORKS
Why your factory uses 192.168 and your phone doesn't.
Estimated 8 minutes.
Not all IPs are equal.
Some IP addresses are reserved for use only inside private networks like your home or factory. Others are "public" — used on the open internet. Knowing the difference is essential.
Three classes you actually meet.
IP addresses were originally divided into 5 classes (A through E). In practice, you will work with three: Class A, B, and C. Each covers a range of addresses for different network sizes.
Class A, B, and C ranges.
A is huge, B is medium, C is small.
The first number tells you.
Look at the first number of any IP. 1–126 is Class A. 128–191 is Class B. 192–223 is Class C. That is the easiest way to spot the class at a glance. Note: 127 is skipped on purpose — 127.0.0.0/8 is reserved for loopback (the address every device uses to talk to itself, like 127.0.0.1).
Classes are historical.
IP classes were the original way the internet was organized, but they were officially replaced in 1993 by a system called CIDR (covered in the Subnet Masks lesson). Modern networks no longer think in classes — they use prefix lengths like /24. We teach classes because you'll still see the terms in documentation and on the plant floor, but in practice the subnet mask is what actually matters.
Classify these.
For each IP below, pick the class it belongs to.
Private vs public.
Within each class, certain ranges are reserved for private use — your home, your factory, your office. The rest is public and used on the internet. Private IPs are free. You can use them behind your network without registering.
The three private ranges.
192.168.x.x is the most common in industry.
Reachable from the internet?
A PLC has IP 192.168.1.10. Is it reachable from the internet?
- Yes
- No
- Only with special configuration
Class + reach, at speed.
A queue of IPs slides in. Pick the class. After 5 correct, the public/private toggle unlocks.
Why factories use 192.168 or 10.x.
Plant floors use private IPs because they are free, plentiful, and isolated from the public internet. A small cell might use 192.168.1.x for its devices. A large facility might use 10.x.x.x because it has room for hundreds of thousands of addresses.
Special addresses.
A few addresses have special meaning. 127.0.0.1 means "this device itself" (localhost). x.x.x.0 is usually the network identifier. x.x.x.255 is usually the broadcast — sends to everyone. Don't assign these to devices.
Pick a safe IP.
You're assigning a static IP to a new HMI on a 192.168.1.x network. Which is safe?
- 192.168.1.0
- 192.168.1.255
- 192.168.1.50
- 127.0.0.1
You can now read any IP at a glance.
- Three useful classes: A (1–126), B (128–191), C (192–223).
- Private ranges: 10.x, 172.16–31.x, 192.168.x.
- Plant floors use private IPs almost exclusively.
- 127.0.0.1, x.x.x.0, and x.x.x.255 are special — don't assign them.
Lesson 03 complete.
- You can identify a class from the first number.
- You know which ranges are private.
- You know which addresses to never assign.