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What are Routers ?

Posted by Richa Sachdeva on

Routers


 Routers are network devices that are used to connect several networks. Routers do not allow broadcasts to be transmitted across different networks it interconnects and hence effectively breaks up a broadcast domain.
Hubs can cause severe degradation of a network because they flood out even the unicast packets. To overcome these, switches are used instead of hubs.

Switches break up collision domain because they provide a single collision domain for every port. 

Every host gets its own collision domain as one host connects to one port on the switch. Thus it eliminates the collisions in the network. 



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Also there is one more benefit if we use switches. Each host can transmit data anytime in a Switched network because Switches simply “switch” the data from one port to another in the switched network. 
Also, unlike hubs, switches do not flood every packet out all ports. They switch a unicast packet to the port where the destination host resides. They only flood out a broadcast packet. 
  • Hubs flood out all packets, even the unicast ones. 
  • Hub does not break collision domains. 
  • Switches break the collision domain
  • Switches do not flood unicast packets.

With Switches Each host can transmit data anytime in a Switched network because Switches simply “switch” the data from one port to another in the switched network. 
Each host is in its own collision domain and if HostA sends a packet to HostC, HostB will not receive it in a Switched network.
Each port of the Switches represent a single collision domain.

Why do we need a Router ?

A switch floods out a broadcast packet to all the hosts because all the hosts connected in a network via Switches are in the same broadcast domain. 

Broadcasts might be essential for network operations but too many broadcasts will slow down the network. 
So we can replace a Switch with a Router to break the broadcast domain.
Thus networks are broken into smaller sizes and these separate networks are interconnected using routers. 
Routers do not allow broadcasts to be transmitted across different networks it interconnects and hence effectively breaks up a broadcast domain. Figure below shows three switched networks which are interconnected by a router. 


In the  Figure above , network broadcasts from hosts connected to Switch1 will not reach hosts connected to Switch2 or Switch3. This is because the router will drop the broadcast on its receiving interface.
In addition to breaking up broadcast domains, routers also perform the following four essential functions in your network:
  • Packet Switching – Routers are like switches because they essentially switch packets between networks(as switches do it between the hosts).
  • Enable Communication between Networks – Routers allow communication between different networks connected to it.
  • Path Selection – Routers talk to each other in order to learn about all the networks connected to them, and then select the best path to reach a network. 
  • Packet Filtering – Routers can drop or forward packets based on certain criteria like their source and destination. 


Points to remember 

  • A hub is used to connect more than 2 hosts . 
  • All hosts in a hub network are in the same collision domains
  • Hubs flood out all packets, even the unicast ones.
  • Hub does not break collision domains. 
  • Switches break the collision domain
  • Switches do not flood unicast packets
  • With Switches Each host can transmit data anytime in a Switched network because Switches simply “switch” the data from one port to another in the switched network. 
  • Switches break collision domains therefore they are more effective than hubs.
  • Switch replaces a hub in a network to make the network more efficient (proper transfer of unicast messages to intended host and allow all the hosts to send messages simultaneously by breaking up the collision domain at each port of the hub)
  • Switched networks interconnected by a router. 
  • All hosts in a Switched network are in same broadcast domain.
  • Routers break broadcast domains 
  • Routers perform Packet Switching between the networks.
  • Routers enable Communication between Networks
  • Routers enable Path Selection
  • Routers enable Packet Filtering

Dear readers, your comments are most welcome. Please feel free to share your ideas.

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