diff --git a/tests/Bifrost.Web.Tests/RegistrationLimitTests.cs b/tests/Bifrost.Web.Tests/RegistrationLimitTests.cs
index 84940f7..cbda9c4 100644
--- a/tests/Bifrost.Web.Tests/RegistrationLimitTests.cs
+++ b/tests/Bifrost.Web.Tests/RegistrationLimitTests.cs
@@ -163,6 +163,34 @@ public sealed class RegistrationBudgetTests(RegistrationBudgetFactory factory) :
         Assert.Equal(HttpStatusCode.OK, other.StatusCode);
     }
 
+    [Fact]
+    public async Task An_IPv6_caller_owns_its_slash_64_and_a_mapped_IPv4_shares_its_IPv4_key()
+    {
+        using var client = factory.CreateClient();
+        string[] sameSlash64 = ["2001:db8:1:2::1", "2001:db8:1:2::2", "2001:db8:1:2:aaaa::3", "2001:db8:1:2:ffff:ffff:ffff:ffff", "2001:db8:1:2::5"];
+        var statuses = new List<HttpStatusCode>();
+        foreach (var peer in sameSlash64)
+        {
+            using var response = await ChallengeAsync(client, peer);
+            statuses.Add(response.StatusCode);
+        }
+
+        Assert.Equal(
+            [HttpStatusCode.OK, HttpStatusCode.OK, HttpStatusCode.OK, HttpStatusCode.TooManyRequests, HttpStatusCode.TooManyRequests],
+            statuses);
+        using var otherSlash64 = await ChallengeAsync(client, "2001:db8:1:3::1");
+        Assert.Equal(HttpStatusCode.OK, otherSlash64.StatusCode);
+
+        for (var i = 0; i < 3; i++)
+        {
+            using var ok = await ChallengeAsync(client, "203.0.113.50");
+            Assert.Equal(HttpStatusCode.OK, ok.StatusCode);
+        }
+
+        using var mapped = await ChallengeAsync(client, "::ffff:203.0.113.50");
+        Assert.Equal(HttpStatusCode.TooManyRequests, mapped.StatusCode);
+    }
+
     [Fact]
     public async Task Challenge_and_register_spend_one_budget_and_a_refused_register_creates_nothing()
     {