Hub
Nerd Cafe
Context:
Four PCs are connected via a hub.
The PC with IP 192.168.1.1 pings 192.168.1.4 (ICMP Echo Request).
A hub is a Layer 1 device: it broadcasts any incoming frame out all other ports.
The images show this process in detail.
1. Initial Topology
All devices are connected to the hub.
Devices and IPs:
PC1: 192.168.1.1
PC2: 192.168.1.2
PC3: 192.168.1.3
PC4: 192.168.1.4
At this point, no traffic is being sent yet.
The network is idle and ready.
2. PC1 sends ICMP Echo Request
PC1 (192.168.1.1) creates an ICMP Echo Request packet.
This packet is encapsulated in an Ethernet frame.
Destination MAC: Broadcast or PC4’s MAC (depending on ARP already resolved or not—but hub doesn’t care).
PC1 sends the frame into the hub.
3. Hub receives the frame
The hub receives the signal on the port connected to PC1.
The hub doesn’t inspect the packet at Layer 2 (it doesn’t learn or filter).
The hub’s role is simple repetition/amplification of the signal.
4. Hub forwards to all ports
The hub forwards the signal to all other ports except the one it received from.
Here:
Signal goes to PC2.
Signal goes to PC3.
Signal goes to PC4.
This is flooding behavior typical of hubs.
5. All PCs receive the frame
All connected PCs receive the Ethernet frame.
But only PC4 (192.168.1.4) will process it:
Checks the destination IP.
Matches its own.
Processes the ICMP request.
PC2 and PC3:
See it’s not for them.
Discard the frame.
6. PC4 prepares ICMP Echo Reply
PC4 creates an ICMP Echo Reply packet.
Encapsulates it in an Ethernet frame with:
Destination MAC of PC1.
Source MAC of PC4.
PC4 sends the frame into the hub.
7. Hub forwards reply to all other ports
Hub receives the frame from PC4.
Hub forwards it to all other ports except PC4’s port:
Sends to PC1.
Sends to PC2.
Sends to PC3.
Result:
PC1 receives the ICMP Echo Reply.
PC2 and PC3 see the frame, see it’s not for them, and discard it.
Summary of Hub Operation in This Scenario
The hub always forwards all frames to all ports (except the incoming one).
No learning, no filtering.
Inefficient compared to a switch.
But it ensures connectivity in simple networks.
In your ping scenario:
Both the request and the reply are broadcast (in terms of the hub’s behavior) to all connected devices.
Only the intended recipient processes the packet.
Cisco Packet Tracer Topology (Version 6.2.0.0052)
Last updated