Anchor IP addresses are IP addresses that reserved IPs can bind to.
Each newly created Droplet receives an anchor IP address, and when you assign a reserved IP to a Droplet, the reserved IP maps traffic to the anchor IP instead of the Droplet’s public IP. The Droplet’s public internet traffic flows between the reserved IP address assigned to it and its dedicated anchor IP address.
Droplets created before October 2015 and Droplets created using custom images do not have anchor IPs assigned to them by default. To assign a reserved IP to a Droplet created before 2015, you must manually add an anchor IP to the Droplet. Droplets created from custom images do not require an anchor IP to use a reserved IP, instead the reserved IP maps to the Droplet’s public IP address by default.