diff --git a/mininet/node.py b/mininet/node.py
index b197a5b..b33346c 100644
--- a/mininet/node.py
+++ b/mininet/node.py
@@ -752,28 +752,32 @@ class Switch( Node ):
     dpidLen = 16  # digits in dpid passed to switch
 
     def __init__( self, name, dpid=None, opts='', listenPort=None, **params):
-        """dpid: dpid for switch (or None to derive from name, e.g. s1 -> 1)
+        """dpid: dpid hex string (or None to derive from name, e.g. s1 -> 1)
            opts: additional switch options
            listenPort: port to listen on for dpctl connections"""
         Node.__init__( self, name, **params )
-        self.dpid = ( ( '0' * self.dpidLen + dpid.translate( None, ':' ) )
-                      [ -self.dpidLen: ] if dpid else self.defaultDpid() )
+        self.dpid = self.defaultDpid( dpid )
         self.opts = opts
         self.listenPort = listenPort
         if not self.inNamespace:
             self.controlIntf = Intf( 'lo', self, port=0 )
 
-    def defaultDpid( self ):
-        "Derive dpid from switch name, s1 -> 1"
-        try:
-            dpid = int( re.findall( r'\d+', self.name )[ 0 ] )
-            dpid = hex( dpid )[ 2: ]
-            dpid = '0' * ( self.dpidLen - len( dpid ) ) + dpid
-            return dpid
-        except IndexError:
-            raise Exception( 'Unable to derive default datapath ID - '
-                             'please either specify a dpid or use a '
-                             'canonical switch name such as s23.' )
+    def defaultDpid( self, dpid=None ):
+        "Return correctly formatted dpid from dpid or switch name (s1 -> 1)"
+        if dpid:
+            # Remove any colons and make sure it's a good hex number
+            dpid = dpid.translate( None, ':' )
+            assert len( dpid ) <= self.dpidLen and int( dpid, 16 ) >= 0
+        else:
+            # Use hex of the first number in the switch name
+            nums = re.findall( r'\d+', self.name )
+            if nums:
+                dpid = hex( int( nums[ 0 ] ) )[ 2: ]
+            else:
+                raise Exception( 'Unable to derive default datapath ID - '
+                                 'please either specify a dpid or use a '
+                                 'canonical switch name such as s23.' )
+        return ( '0' * self.dpidLen + dpid )[ -self.dpidLen : ]
 
     def defaultIntf( self ):
         "Return control interface"