Loopback

The loopback CIDR is 127.0.0.0/8.

Loopback is Host-Specific, Not Network-Specific

Key Point: Loopback addresses operate at the host level, not the network level. Every device has its own local loopback interface.

How Loopback Works in Different Networks

Small Network (e.g., 192.168.1.0/24):

Router: 192.168.1.1 (also has 127.0.0.1 loopback)PC1: 192.168.1.10 (also has 127.0.0.1 loopback)  PC2: 192.168.1.11 (also has 127.0.0.1 loopback)

Large Network (e.g., 10.0.0.0/16):

Server: 10.0.1.100 (also has 127.0.0.1 loopback)Workstation: 10.0.2.50 (also has 127.0.0.1 loopback)

Loopback Network Details

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

Most Common Loopback Address

127.0.0.1 - The standard "localhost" address

Key Characteristics

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)

Examples of Loopback Usage

bashping 127.0.0.1          # Test local TCP/IP stackcurl http://127.0.0.1   # Connect to local web serverssh 127.0.0.1           # SSH to local machine

AWS Context

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 Standard

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.