This video is the third video in our ongoing tutorial series on CCNA. This video explains unicast, broadcast, and Address Resolution Protocol.
Video description
We have a simple network of four PCs. PC1 wants to send a file to PC3. For this, PC1 needs to know the IP address and MAC address of PC3. PC1 can learn the IP address of PC3 in two ways. It can use the hosts file or a DNS server. A hosts file is a text file. Almost all operating systems have it. It allows us to map IP addresses to names. If this file does not contain an entry for a name, the system uses the DNS server to translate the name.
From the hosts file entries, PC1 learns that PC3's IP address is 10.0.0.3/8.
After knowing PC3's IP address, it needs to know the MAC address of PC3. It uses the ARP protocol to learn the MAC address of PC3. ARP protocol uses a simple mechanism to know the MAC address of the given IP address. It sends a broadcast message to the network, assuming the device with this IP address will listen and reply. The broadcast message includes the source's MAC address, IP address, request message, and intended device's IP address. The broadcast message reaches all devices in the network. A recipient device matches its IP address with the IP address listed in the indented IP address field of the broadcast message to determine whether this message is for it.
This broadcast message is for the device with the IP address 10.0.0.3/8. PC2 and PC4 ignore this message as their IP addresses do not match with it. Only PC3 replies to this message as its IP address matches the IP address listed in the indented IP address field of the broadcast message. Since this message also includes the PC1's MAC and IP addresses, PC3 does not need a broadcast message to reply. It can send a direct message to PC1. A direct message is known as a unicast message. PC3 sends a unicast reply message to PC1. The reply message includes PC3's MAC address. PC1 learns the PC3's MAC address from the reply message. After learning PC3's MAC address, it sends the file directly to PC3 using a unicast message.
This mechanism explains three things: broadcast, unicast, and ARP protocol.
Broadcast
A broadcast message is for all. It contains pre-defined IP and MAC addresses in the destination IP and MAC address fields.
These IP (255.255.255.255) and MAC (FFFF.FFFF.FFFF) addresses belong to all devices in the network, and all devices listen to them.
Devices use their logic to determine whether to reply to it.
The sending device does not need to know the intended device's IP and MAC address.
Unicast
A unicast message is for a particular device.
It contains the IP and MAC addresses of the intended device in the destination IP and MAC address fields.
A device can send a unicast message only when it knows the IP and MAC addresses of the destination device.
ARP
ARP stands for Address Resolution Protocol.
An end device uses it to find the MAC address associated with an IP address.
It sends an ARP broadcast to know the MAC address associated with an IP address.
It contains the source device's MAC and IP addresses in the source field, the intended IP address, and the broadcast IP and MAC addresses in the destination field.
All devices on the local network listen to it.
They compare the intended IP address with their IP address to determine whether it belongs to them.
Only the device whose IP address matches the intended IP address replies to the message. All other devices ignore it.
The matched device sends a unicast message to the source device.
The message contains the MAC address associated with the intended IP address.
The source device learns the MAC address from the message and saves it in the ARP cache for later use.
The source device sends an ARP broadcast to know the MAC address associated with an IP address. The broadcast message contains the source device's MAC and IP addresses in the source field, the intended IP address, and the broadcast IP and MAC addresses in the destination field.
Since the broadcast message contains the IP and MAC addresses that belong to all, all devices listen to it. They compare the intended IP address with their IP address to determine whether it belongs to them. Only the device whose IP address matches the intended IP address replies to the message. All other devices ignore it. The matched device sends a unicast message to the source device. The message contains the MAC address associated with the intended IP address. The source device learns the MAC address from the message. It saves this address in the ARP cache for further use. If it gets a data packet for the same IP address again, it uses the cache entry instead of repeating this process.