ICMP (Request and reply)

Whenever a host joins a network, the first thing it should know is, what is the IP address of its default router. To find that out it sends an ICMP packet to all the routers it is connected to. Whichever router wants to be its default router sends back another ICMP, containing its IP address.

Whenever a router joins a new network, it advertises itself by sending an ICMP packet to all the hosts in the network. It says that I am a new router in the network if anyone wants to make me their default router they are welcome.

If a host in the network doesn't know it's network mask, it can use ICMP to find it out. The host can send an ICMP packet to its default router and default router sends back another ICMP, which contains the network mask of the host.

Whenever two hosts are present at two different parts of the world, it's obvious that there could be some synchronisation issue related to time. One of the hosts can send an ICMP to other host and can find out the time onto other hosts machine. Both can synchronise their time clock to communicate properly.

This technique is not used anymore. New message protocols are used now. Which are better than this.

There is one more application of ICMP Req & reply, Traceroute. I have covered this topic in-depth in another post. You can read about it here.

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