diff -r eaae4008327d Lib/ipaddress.py
--- a/Lib/ipaddress.py	Tue Feb 04 09:49:14 2014 +0100
+++ b/Lib/ipaddress.py	Tue Feb 04 13:00:59 2014 +0000
@@ -598,12 +598,14 @@
         broadcast = int(self.broadcast_address)
         if n >= 0:
             if network + n > broadcast:
-                raise IndexError
+                raise IndexError('%d out of range 0..%d for %s'
+                                 % (n, self.num_addresses - 1, self))
             return self._address_class(network + n)
         else:
             n += 1
             if broadcast + n < network:
-                raise IndexError
+                raise IndexError('%d out of range -%d..-1 for %s'
+                                 % (n - 1, self.num_addresses, self))
             return self._address_class(broadcast + n)
 
     def __lt__(self, other):