In Mininet (up to 2.1.0 at least) you cannot have multiple Mininet instances running without running into namespace conflicts for the nodes and port conflicts for local controllers. This example demonstrates an easy way to specify a unique prefix for the nodes and a unique port for the controller, thus allowing multiple Mininet instances to coex…

prefix.py

#!/usr/bin/python

"""
Example of adding a node prefix to avoid namespace conflicts
(and changing the controller port to avoid port conflicts.)

If we use UserSwitch, then we can run in a Mininet host (!)
"""

from mininet.net import Mininet
from mininet.topo import Topo, LinearTopo
from mininet.node import Controller, UserSwitch
from mininet.cli import CLI
from mininet.log import setLogLevel

from sys import argv, exit
from functools import partial

class PrefixTopoMixin( Topo ):
    "Mix-in for adding node prefixes"
    prefix = ''
    def addNode( self, name, **opts):
        name = self.prefix  + name
        return super( PrefixTopoMixin, self).addNode( name, **opts )

class PrefixLinearTopo( PrefixTopoMixin, LinearTopo ):
    "Linear topo with node prefixes"
    pass

try:
    prefix = argv[ 1 ]
    port = int( argv[ 2 ] )
except:
    print 'usage: %s <node-prefix> <controller-port>' % argv[ 0 ]
    exit( 1 )

setLogLevel( 'info' )
PrefixLinearTopo.prefix = prefix
net = Mininet( topo=PrefixLinearTopo( 3 ), switch=UserSwitch,
               controller=partial( Controller, port=port ) )
net.start()
CLI( net )
net.stop()

This is just off the top of my head - it could certainly be improved in a variety of ways.