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IPv4 Classes (A to E) and Loopback
IPv4 Classes (A to E) and Loopback Old IPv4 addresses were split into classes A to E by the first number. Classes are history now, but you still see the ranges. 127.0.0.1 is loopback : a computer talking to itself. A service on 127.0.0.1 cannot be reached from outside. 127.0.0.1 in a web log can mea
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What this card says
What it is
Old IPv4 addresses were split into classes A to E by the first number.
Classes are history now, but you still see the ranges.
Loopback
127.0.0.1 is loopback: a computer talking to itself.
A service on 127.0.0.1 cannot be reached from outside.
Why it matters for security
127.0.0.1 in a web log can mean a normal reverse proxy.
It can also mean SSRF: an attacker tricks the server into calling itself.
Keep MySQL on 127.0.0.1 so the internet cannot reach it.
Common mistake
Class A is not 0–127.
0 and 127 are special, so normal Class A is 1–126.
| Class | First octet range | Leading bits |
|---|---|---|
| A | 1 – 126 | 0 |
| B | 128 – 191 | 10 |
| C | 192 – 223 | 110 |
| D | 224 – 239 | 1110 |
| E | 240 – 255 | 1111 |
Learn more in the lesson: 1.3 IPv4 Classes (A to E) and Loopback