Feature Request: Add IPv4 utility functions (Validation, class detection, hex -> dotted, dotted -> hex)

I'd like to contribute IPv4-related utility functions written in C, including:

- Validating IPv4 class (A/B/C/D/E)
- Conversion between dotted-decimal notation and hexadecimal

## Motivation

These features could help in networking-related C modules or teaching tools. They are self-contained and easy to integrate into existing code bases or libraries.