ARP: Address Resolution Protocol (L2)
When sending a ping to a device connected to a switch, an ICMP (Internet Control Message Protocol) is sent to that destination IP as well as an ARP packet. This is only done when the switch does NOT have the L2 MAC address stored for the destination IP address. Normally it should just send the ICMP packet.
A device will send a Broadcast Address (FFFF.FFFF.FFFF) ARP packet when trying to find the MAC address associated with another device’s IP.
When trying to connect to an IP Address outside a device’s configuration (ex. 10.1.1.0/24 → 23.93.29.392) the device will send an ARP packet to the router’s default gateway. (ex. 10.1.1.5 → 10.1.1.1)
An Example of this in Cisco Packet Tracer
You CANNOT hook up two different networks together using a switch. It will not work.
Routers use Layer 3 to communicate
When sending a DNS query from a device, the process of the switch, router, and device finding each other via ARP starts and a request is made. The DNS server replies with the IP information regarding the website and sends a HTTP GET (more on this is in later sections) request to the routers default gateway and out the internet.
When looking under the hood inside a router it stores a map of all different IP networks and how it connects to all of them. This can be seen in Cisco Packet Tracer using the command show ip route
