Question

While sending an IP datagram from host A to host B and a return a datagram from host B to host A, where A and B are on different networks . While sending an IP datagram from host A to host B and a return a datagram from host B to host A, where A and B are on different networks .

a) What will be the source and destination IP addresses on the datagram sent from host A? b) What will be the destination MAC address on the frame containing the datagram sent by host A? c) Which address will change by the time the information reaches host B and which will ones will remain th same? d) What will be the source and destination IP addresses on the datagram sent from host B ? e) What will be the destination MAC address on the frame containing the datagram sent by the host B? f) Which address will change by the time the information reaches host A and which will remain the same?

Answer 1

a)Host A will send a Datagram packet with Source IP Address of A itself & destination IP address of host B.

b)The destination MAC address on the frame containing the datagram sent by host A is MAC address of Default Gateway which is in most cases is a Router.

c) The MAC address of source & destination will change by the time the information is reached to host B (Layer 2) , whereas the Source & destination IP address(Layer 3) will remain the same.

d)Since,Layer 3 provides Connectionless Service unlike Layer 2 so Host B will send a Datagram packet with Source IP Address of B & destination IP address of host A.

e)The destination MAC address on the frame containing the datagram sent by host B is MAC address of Default Gateway directly attached to host B which is in most cases is a Router.

f) The MAC address of source & destination will change by the time the information is reached to host A (Layer 3) , whereas the source & destination IP address(Layer 3) will remain the same.

Explanation on How Communication takes place between two hosts on different network:

When Communication takes place between two hosts on different network, layer 2 device (Switch) cannot be used for inter-communication because Switch works on MAC address and MAC addresses are obtained via broadcast of ARP request and we cannot send a broadcast in whole world to find a single machine. To solve this problem,Layer 3 addressing was developed and Layer 3 device such as Router is used for inter-communication.
The Router makes use of IP Addresses.