Hello Hussein.
You are correct when you say that you cannot skip an OSI layer when communicating on the network. However, we can BEGIN our communication at layer 3 and go down to layer 1. In this case we are not skipping layers 4-7. Let me express this in an example:
When you start an FTP file transfer from your computer, you are BEGINNING your communication at the Application layer, or layer 7. As you go down the OSI stack, you cannot skip layer 2 for example. MAC addresses must be placed in the L2 header and appropriate header information must be included. You can’t get to layer 1 otherwise. All 7 layers must be traversed because you’re starting from layer 7. It’s like getting in an elevator on the 7th floor and you want to go to the ground floor. You can’t skip floor 2!!
However, if you happen to be on the third floor, you can enter the elevator and go to the ground floor. You’re not skipping floors 7-4, you just happen to be starting at floor 3 and going down. That’s what ICMP is doing.
Similarly, ARP is a layer 2 protocol and does not know layers 3-7 exist. Similarly routing protocols (EIGRP, RIP, OSPF etc) all operate at layer 3. CDP, VTP and LACP are examples of protocols that exist only on layer 2 and know nothing of upper layers.
Using the elevator analogy, you can see that we are not skipping layers, we are just getting on the elevator at a different “floor” to start our journey down.
I hope this has been helpful!
Laz