From c935fa152a2d4f36ad755845fba77da1435ab989 Mon Sep 17 00:00:00 2001
From: Otthorn <otthorn@crans.org>
Date: Thu, 15 Apr 2021 00:35:04 +0200
Subject: [PATCH] :bug: Fix serial number arithmetics

---
 lib.py | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/lib.py b/lib.py
index 091006a..031019a 100644
--- a/lib.py
+++ b/lib.py
@@ -87,10 +87,11 @@ def update_serial(serial, serial_bits=32):
     """Update serial number
 
     According to RFC 1982 and Knot implementation.
-    SERIAL_BITS = 32 by default.
+    SERIAL_BITS = 32 by default, which means the serial number counter can
+    range from 0 to 2^32 - 1.
     """
 
-    serial = serial + 1 % 2**32
+    serial = (serial + 1) % 2**serial_bits
 
     return serial