router(config)#ip route 172.16.0.0 255.255.0.0 S0/0/0
router(config)#ip route 172.17.0.0 255.255.0.0 S0/0/0
router(config)#ip route 172.18.0.0 255.255.0.0 S0/0/0
router(config)#ip route 172.19.0.0 255.255.0.0 S0/0/0
router(config)#ip route 172.20.0.0 255.255.0.0 S0/0/0
router(config)#ip route 172.21.0.0 255.255.0.0 S0/0/0

===========the router routing table will contain 7 routes

===========we will try to summarize the routes

172.(00010 000).0.0/16
172.(00010 001).0.0/16
172.(00010 010).0.0/16
172.(00010 011).0.0/16
172.(00010 100).0.0/16
172.(00010 101).0.0/16
===================
172.(00010 000).0.0/13
172.16.0.0/13

Be careful: we also summarize 172.(00010110).0.0/16 and 172.(00010111).0.0/16
Be careful: we also summarize 172.22.0.0/16 and 172.23.0.0/16

if not used in other site within the network the summary is accepted
	router(config)#ip route 172.16.0.0 255.248.0.0 S0/0/0
else we need to change the summary

	172.(000100 00).0.0/16		172.(0001010 0).0.0/16
	172.(000100 01).0.0/16		172.(0001010 1).0.0/16
	172.(000100 10).0.0/16		======================
	172.(000100 11).0.0/16		172.(0001010 0).0.0/15
	======================		172.20.0.0/15
	172.(000100 00).0.0/14
	172.16.0.0/14
	
	router(config)#ip route 172.16.0.0 255.252.0.0 S0/0/0
	router(config)#ip route 172.20.0.0 255.254.0.0 S0/0/0