Use the wildcard-mask option with the network command to advertise only the subnet and not the
entire 192.168.20.0 classful network.

Note: Think of a wildcard mask as the inverse of a subnet mask. The inverse of the subnet mask
255.255.255.240 is 0.0.0.15. To calculate the inverse of the subnet mask, subtract the subnet mask from
255.255.255.255:

255.255.255.255
– 255.255.255.240   Subtract the subnet mask
-------------------
0 . 0 . 0 . 15   Wildcard mask

R1(config-router)# network 192.168.20.4 0.0.0.15

R1(config-router)#