Windows computers communicating over a HUB

[Cisco10956]

HUBI have 5 Windows 7 computers all connected to an eight port HUB. 

PC1 wants to send a message to PC3;   The person on PC1 pings the ip address of PC3. 

The first time, the message is sent to all "broadcast " computers connected to the HUB, then PC3 determines its the correct computer to accept, and all others discard the message. 

The second time same request is made to PC3 from PC1. 

   Question : Will this response be unicast and not broadcast because PC1 has learned the MAC address and ip address of PC3.  - Is this correct.  PC1 has stored this information in it's cache

[Jon Marshall · Accepted solution]

Yes, as long as it the entry has not timed out of the arp cache as explained in your other post. 

Jon

[Kasun Bandara]

even though PC3 sends message only to PC1 MAC address, HUB will send that to all ports because HUB dont know about any MAC address to select port. HUB dont have intelligence to learn MAC addresses or any information above layer 1.

Answer to Q : PC3 will send only to PC1s MAC address if its MAC cache is not expired.

[Richard Burts]

The original post contains a fundamental mistake about how IP networking works: "PC1 wants to send a message to PC3; The person on PC1 pings the ip address of PC3." PC1 does not ping PC3. To send a data packet to PC3 you need both the destination IP address and the destination mac address. PC1 knows the destination IP but does not know the destination mac. So PC1 sends an arp request for PC3. The arp request is sent as a broadcast and the hub forwards a copy of the request to all of its ports. PC3 receives the arp request and sends an arp response which contains its mac address. Now that PC1 knows both the destination IP and the destination mac so it creates an entry in its arp table. Now it can send the data packet to PC3. Note that all devices connected to the hub received a copy of the arp request. All devices other than PC3 realize that the arp request was not for them and ignore it.

For the second packet from PC1 to PC3 there is already an entry in the arp table and so PC1 simply sends a unicast IP packet to PC3. There is no need for additional arp request for PC3 (until the arp table entry ages out and in that case there will need to be another arp request).