1. Hub vs Switch
Both let LAN end hosts (PCs, servers, printers) plug in over Ethernet and talk to each other. The difference is the layer they operate at and how much they know.
Hub
Switch
Operates at
Layer 1
Layer 2 (and Layer 1)
MAC addresses
Not aware
Aware
Duplex
half-duplex only
full-duplex capable, always full in practice
Simultaneous send/receive
No, one or the other
Yes
Collision Domain
All hosts share one
Dedicated per port
Collision Detection
Required, CSMA/CD detects then retransmits
Not required
Frame handling
Floods every port except the one it came in on
Sends to the destination port only (once learned)
2. How a Switch Works
Step
Behavior
Learning
On receiving a frame, record the Source MAC from the Ethernet header against the receiving port in the MAC address table
Forwarding
If the Destination MAC is in the table, send only out that port
Flooding
If the destination is a broadcast or an unknown unicast, send out every port except the one it arrived on
There are only two flooding conditions: broadcast, or unknown unicast.
Sending only to the destination port helps both performance and security. The frame goes where it’s needed, so other hosts don’t spend cycles on it and never see traffic that isn’t theirs. A hub does the opposite: it sprays every port, so every host has to process every frame.
3. Router
- Knows the paths to the various IP subnets on the network
- Sending traffic from one subnet to another always requires a router
- Operates at Layer 3 (also at Layers 2 and 1, and usually aware up to Layer 7)
Router
Switch
Aware at
Layer 3
Layer 2
Role
Routing between different networks
Switching between hosts on a LAN
Interfaces
Varied: Ethernet, Serial, ISDN, ADSL
Usually Ethernet only
Port count
Few
Many
Broadcasts
Not forwarded by default
Forwarded
Hosts in the same subnet (10.10.10.x) talk through a switch alone, but the moment another subnet (10.10.11.x) is involved a router is required.
Layer 3 Switch
- Higher-end switches are Layer 3 aware and can route between IP subnets
- Interfaces are still Ethernet only, and port count is higher than a router’s
- Typical build: a Layer 3 Switch routes between internal subnets, a router handles the WAN
A Layer 3 Switch is a switch with router functions.
4. Summary
- Hub is L1, half-duplex, one collision domain, no MAC awareness → always floods
- Switch is L2, full-duplex, per-port collision domains, keeps a MAC address table
- Learning uses the Source MAC, forwarding uses the Destination MAC
- Flooding happens on broadcast or unknown unicast (out every port but the ingress one)
- Many MACs per port is fine, one MAC on many ports is not
- A router is required for inter-subnet traffic. Varied interfaces, few ports, no broadcast forwarding
- A Layer 3 Switch can route but has Ethernet interfaces only