diff --git a/frontend/http/parser.go b/frontend/http/parser.go
index d873f40..33a499d 100644
--- a/frontend/http/parser.go
+++ b/frontend/http/parser.go
@@ -79,6 +79,11 @@ func ParseAnnounce(r *http.Request, realIPHeader string, allowIPSpoofing bool) (
 		return nil, bittorrent.ClientError("failed to parse peer IP address")
 	}
 
+	// Sanitize IPv4 addresses to 4 bytes.
+	if ip := request.Peer.IP.To4(); ip != nil {
+		request.Peer.IP = ip
+	}
+
 	return request, nil
 }