diff --git a/botgate.md b/botgate.md
index 87cebc4..e5d0063 100644
--- a/botgate.md
+++ b/botgate.md
@@ -239,6 +239,8 @@ https://www.ip2location.com/free/visitor-blocker
 
 When downloading, select **"Apache 2.0 - 2.3 .htaccess deny"** as the output format. Other formats offered on that page (Apache 2.4, Nginx, CIDR, iptables, etc.) are not recognized by BotGate's parser.
 
+Choose **IPv4** lists. BotGate listens on IPv4 only, so IPv6 ranges in a geo file are skipped with a warning.
+
 IP2Location recommends refreshing these lists monthly, since IP allocations change over time.
 
 ### Performance
diff --git a/botgate_proxy.py b/botgate_proxy.py
index fcfdb70..2f9d730 100644
--- a/botgate_proxy.py
+++ b/botgate_proxy.py
@@ -415,6 +415,7 @@ def load_geo_file(path):
     ends) for fast binary-search containment checks. Non-'deny from'
     lines (comments, <Limit>, order/allow directives) are ignored."""
     ranges = []
+    skipped_ipv6 = 0
     try:
         with open(path, "r", errors="ignore") as f:
             for line in f:
@@ -427,9 +428,17 @@ def load_geo_file(path):
                 except ValueError:
                     log.warning(f"Skipping malformed entry in {path}: {cidr!r}")
                     continue
+                if net.version != 4:
+                    # Listeners are IPv4-only, and an IPv6 range does not fit in
+                    # the array below (OverflowError at startup).
+                    skipped_ipv6 += 1
+                    continue
                 ranges.append((int(net.network_address), int(net.broadcast_address)))
     except OSError as e:
         log.warning(f"Could not read geo file {path}: {e}")
+    if skipped_ipv6:
+        log.warning(f"Skipped {skipped_ipv6} IPv6 range(s) in {path}: "
+                    f"BotGate listens on IPv4 only.")
 
     ranges.sort()
     starts = array.array("L", (r[0] for r in ranges))
diff --git a/tests/test_geo.py b/tests/test_geo.py
new file mode 100644
index 0000000..b3c3f84
--- /dev/null
+++ b/tests/test_geo.py
@@ -0,0 +1,28 @@
+"""Geo blocklist loading."""
+from pathlib import Path
+import sys
+import tempfile
+import unittest
+
+sys.path.insert(0, str(Path(__file__).resolve().parents[1]))
+import botgate_proxy as botgate
+
+
+class GeoFileTests(unittest.TestCase):
+    def test_ipv6_ranges_are_skipped_not_fatal(self):
+        with tempfile.TemporaryDirectory() as directory:
+            path = Path(directory) / "mixed.txt"
+            path.write_text("deny from 203.0.113.0/24\n"
+                            "deny from 2001:db8::/32\n"
+                            "deny from 198.51.100.0/24\n")
+            with self.assertLogs(botgate.log, level="WARNING") as logs:
+                starts, ends = botgate.load_geo_file(str(path))
+        self.assertEqual(len(starts), 2)
+        for ip in ["203.0.113.7", "198.51.100.200"]:
+            self.assertTrue(botgate.geo_range_contains(int(botgate.ipaddress.ip_address(ip)), starts, ends), ip)
+        self.assertFalse(botgate.geo_range_contains(int(botgate.ipaddress.ip_address("192.0.2.1")), starts, ends))
+        self.assertTrue(any("1 IPv6 range" in line for line in logs.output), logs.output)
+
+
+if __name__ == "__main__":
+    unittest.main()