diff --git a/nix/pkgs/subnetplan/render.rb b/nix/pkgs/subnetplan/render.rb
index 47211c8..5ef9772 100644
--- a/nix/pkgs/subnetplan/render.rb
+++ b/nix/pkgs/subnetplan/render.rb
@@ -60,7 +60,11 @@ exit 1 if collisions > 0
 GROUP_PREFIX = 19
 groups = {}
 nets.each do |net|
-  group = net.addr.supernet(GROUP_PREFIX).to_s
+  if net.addr.prefix > GROUP_PREFIX
+    group = net.addr.supernet(GROUP_PREFIX).to_s
+  else
+    group = net.addr.to_s
+  end
   (groups[group] ||= []) << net
 end