PING Command

Question

When echo request is sent ,then internally "ARP request"is generated  for "getting destination MAC " , but what is the need of destination MAC ,if the IP assigned for every host is already unique ! The reply whatever is sent after getting the dest. MAC that can be sent without knowing MAC also ...??? Please clear this doubt

Answer 1

Echo Request message is actually a ICMP Packet which is a Network Layer Protocol.Let station A sends Echo request at Network layer of A  to station B (its network layer).So A need to have B's MAC address (using ARP request) because when A sends this ICMP request it is unwrapped at A's DataLink Layer(here A's datalink layer must knows the datalink address of B which is MAC address of B) and furthur unwrapped at A's physical layer and then is send to B's physical layer then upper to B's Datalink layer and then upper to B's network layer which is its destination.the response message follows same procedure to send Response message to A's Network layer.

Note-:Each layer need to know each layer address respectively to connect to each other as TCP/IP follows peer to peer connection.

Comments

Thank you

Answer 2

IP layer addressing which is hierarchical routing works  only if its inter-network routing. IP addressing doesnt work within the same network.

The reason is because:

1) legacy. MAC addresses were used primarily for addressing within the same network.

2) Lets say you try to identify a host by IP address. But before that packet passes to network layer, it will be discarded by the data link layer. So, the arrangement of layers is another reason.

Thats why even if we use IP, we need MAC addresses on top of it.