Address Resolution Protocol (ARP)

What is ARP broadcast ?

A well known and common term ARP Broadcast in the network industry, is simply stands for Address Resolution Protocol. knowing the following terms are also will be helpful to understand the functions of this protocol.

Broadcast - Send to many
Multicast   - Send to many, but not to all
Unicast     - Send to specific destination

Address Resolution Protocol is a something to gather data about destination MAC address before starting to send data packets. The method to find out the destination's MAC address is send broadcast messages to all the connected devices with requesting their MAC addresses. As seems as the name, In the ARP broadcast, A data packet will be sent to the all connected devices within the network. The word "within the network" is important, because the ARP broadcast message will never go beyond the router. that usually means only in local network. 

There's a reason to not to allow ARP broadcasts beyond the router. because such messages are making remarkable traffic on network. Since the every drop of bandwidth is very precious on internet, such traffic making packets are restricted to go beyond router. 

Now let's see how is the Address Resolution Protocol is functioning.
we'll just consider the following scenario

Think that the HOST A wants to send some data to HOST B. But HOST A still don't know the MAC address of the HOST B. Then before starting to send data packets, The HOST A sends a ARP broadcast message to request the HOST B's MAC address. All devices in the local network receive the ARP and as same as HOST B is also receive the ARP Broadcast from HOST A because it's also in the same network. Then HOST B sends a uncast ARP reply message to HOST A including his MAC address. The word "unicast" is important because HOST B know where to send the reply and he directly send it to HOST A. After that the HOST A starting to send data to HOST B.

Let's look at a little bit different situation like this.

Think, in the above network, HOST A wanted to send some data to HOST B. But keep in mind the HOST A and HOST B are on different networks current situation because a router is in between them. You already know that the ARP broadcast messages are not passes through the router. 

Now what happen is, Router knows the HOST B's MAC address. when the ARP broadcast gets to the router, the router sends a message to the HOST A like this. "The ARP broadcast can't be allow through me, but the destination you are looking for may be outside the network. so, this is the MAC address of the default gateway (ethernet 0). please send data to here and I'll send them to the correct destination on behalf you."

Then the data transferring is taking place.

Important.
ARP messages are broadcasts
Successfully forwards through Switches and hubs
Not allow beyond or through router

Basic Switching - Switch Functions

Basic Switch Functions

In order to forward a packet that has arrived at a router interface, the router must perform a switching function. This switching function has four steps:

• A packet transiting the router will be accepted into the router if the frame header contains the MAC address of one of the router's NIC cards. If properly addressed, the frame and its content will be buffered occurs in memory pending further processing.

• The switching process checks the destination logical network portion of the packet header against the network/subnetwork entries in the routing table. If the search is successful, the switching process associates the destination network with a next-hop logical device and an outbound interface.

• For the next device in the relay chain. The lookup is performed in an Address Resolution Protocol (ARP) table for LAN interfaces or a map table for WAN interfaces.

• Once the physical address of the next-hop device is known, the frame header is overwritten, and the frame is then moved to the outbound interface for transmission onto the media. As the frame is placed on the media, the outbound interface adds the CRC character and ending delimiters to the frame. These characters will need to be validated at the arriving interface on the next-hop relay device.