Routes users to different servers based on their IP address. You tell Route 53: "if a user's IP is in this range, send them to this server."

Why Use It?

Key Concepts

CIDR — A Range of IP Addresses

CIDR is just a way to write a block of IPs in shorthand.

203.0.113.0/24  →  all IPs from 203.0.113.0 to 203.0.113.255  (256 addresses)
200.5.4.0/24    →  all IPs from 200.5.4.0   to 200.5.4.255    (256 addresses)

Think of /24 as one street (256 houses) and /16 as an entire neighborhood (65,536 houses).

CIDR Collection

A named list of IP ranges and their locations. You create this in Route 53.

CIDR Collection: "my-isps"
  Location-1  →  203.0.113.0/24
  Location-2  →  200.5.4.0/24

User-IP-to-Endpoint Mapping (DNS Record)

A DNS record that links a location name to an actual server IP.

example.com  →  1.2.3.4  (IP-based: location-1)
example.com  →  5.6.7.8  (IP-based: location-2)

How It Works — Step by Step

User A (IP: 203.0.113.56)               User B (IP: 200.5.4.100)
         │                                        │
         └──────────────┬─────────────────────────┘
                        ▼
                    Route 53
                        │
              Check CIDR Collection
                        │
         ┌──────────────┴──────────────┐
         ▼                             ▼
  Matches Location-1           Matches Location-2
  (203.0.113.0/24)             (200.5.4.0/24)
         │                             │
         ▼                             ▼
  Server 1.2.3.4               Server 5.6.7.8
  (New York)                   (London)

Step 1 — You create a CIDR Collection mapping IP ranges to location names.

Step 2 — You create DNS records linking each location to a server IP.

Step 3 — User visits your site. Route 53 sees their IP.

Step 4 — Route 53 looks up the IP in your CIDR Collection, finds the matching location, and returns the correct server IP.