Please, suggest best resources for network designe !

Hello,

Most courses are free, some are certified too.

You can do many courses for network designing for example:

• Introduction to Networking for Complete Beginners( it's free on udemy APP)
• How to Build your Professional Network( udemy)
• Cisco CCNA Networking Basics for Beginners: Getting Started
• Networking Concepts for Beginners
• Computer Science 101 - Computers & Programming for Beginners
• The Complete Networking Fundamentals Course
• Networking Foundations: IP Addressing
• Networking Foundations: IP Addressing

You can visit Coursera,udemy for learning Networking for free or for a minimal amount.

Secondly there is no better way to learn technology than getting your hands dirty. To learn about networking, it's important you understand what all are the scenarios in which devices can be connected to each other and how would you make them talk.

So for doing so you can download any network simulator(like GNS3) and build a network of your own.

Hope this helps!!