Host A on Network A can communicate with all other hosts on Network A and with Router R. To communicate with local hosts and the interface of Router R (which are all in the same subnet) only a correct IP address is reqired. If the default gateway of Host A is incorrect, then it will not be able to communicate with any host on the other side of the router, which includes Network B in the diagram. Packets from hosts on Network B will reach Host A on Network A without any problem, because they possess the correct address of the default gateway or router, but Host A will send the packet to a dead end because Host A has an incorrect default gateway. On the other hand, Host A does not require a default gateway to communicate with other hosts on same network.

Host C on Network A WILL be able to communicate with Host A on Network A , even though Host A has an incorrect default gateway because Host A and C are in the same subnet, which requires no use of the of the gateway or router.

Host C on Network A WILL be able to communicate with Router R because Host C has the correct default gateway address which is the address of Router R.

Host D on Network B WILL be able to communicate with Host B on Network A because both hosts have a correct default gateway address.