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The loopback CIDR is 127.0.0.0/8.
Key Point: Loopback addresses operate at the host level, not the network level. Every device has its own local loopback interface.
Small Network (e.g., 192.168.1.0/24):
Large Network (e.g., 10.0.0.0/16):
CIDR Range: 127.0.0.0/8 IP Range: 127.0.0.0 to 127.255.255.255 Subnet Mask: 255.0.0.0 Total Addresses: 16,777,216 addresses
127.0.0.1 - The standard "localhost" address
Purpose:
Internal host communication
Testing and diagnostics
Applications talking to themselves
Behavior:
Traffic never leaves the local machine
No physical network interface required
Always available (even without network connectivity)
In AWS, you'll encounter loopback when:
EC2 instances use 127.0.0.1 for local services
Container networking (Docker uses 127.0.0.1 inside containers)
Application configuration (databases, web servers binding to localhost)
RFC 5735 officially defines 127.0.0.0/8 as:
"Loopback addresses"
Reserved for internal host loopback communication
Must not appear on any network
Bottom line: 127.0.0.0/8 is the complete loopback CIDR range, with 127.0.0.1 being the most commonly used address within that range.