What is Azure Network Security Group (NSG) and how does it work?

Azure NSG contains security rules that allow or deny network traffic to resources in a virtual network. It's similar to a firewall that filters traffic based on:

• Source/Destination: IP addresses, service tags, application security groups
• Protocol: TCP, UDP, ICMP, ESP, AH, or Any
• Port ranges: Specific ports or ranges
• Direction: Inbound or outbound

Key Features:

• Default rules: Cannot be deleted, lowest priority
• Custom rules: Priority 100-4096, lower numbers processed first
• Service tags: Predefined groups (Internet, VirtualNetwork)
• Application Security Groups: Logical grouping of VMs

Example NSG Rule:

Priority: 100
Name: Allow-SSH
Source: 203.0.113.0/24
Destination: Any
Service: SSH (22)
Action: Allow