Hubs, Bridges, Switches and Routers are used to build networks. If you are trying to design your own LAN (Local Area Network) at home, then you probably need to know what they do and the main differences between them. I will try to cover all that in addition to some networking details to cultivate the article and provide better understanding of how the internet works. After all, always remember that the internet as you know it is nothing more than a network of networks!
Hubs are used to build a LAN by connecting different computers in a star/hierarchal network topology, the most common type on LANs now a day. A hub is a very simple (or dumb) device, once it gets bits of data sent from computer A to B, it does not check the destination, instead, it forwards that signal to all other computers (B, C, D…) within the network. B will then pick it up while other nodes discard it. This amplifies that the traffic is shared.
There are mainly two types of hubs:
1. Passive: The signal is forwarded as it is (so it doesn’t need power supply).
2. Active: The signal is amplified, so they work as repeaters. In fact they have been called multiport repeaters. (use power supply)
Hubs can be connected to other hubs using an uplink port to extend the network.
OSI Model: Hubs work on the physical layer (lowest layer). That’s the reason they can’t deal with addressing or data filtering.
Switches on the other hand are more advanced. Instead of broadcasting the frames everywhere, a switch actually checks for the destination MAC address and forward it to the relevant port to reach that computer only. This way, switches reduce traffic and divide the collision domain into segments, this is very sufficient for busy LANs and it also protects frames from being sniffed by other computers sharing the same segment.
They build a table of which MAC address belongs to which segment. If a destination MAC address is not in the table it forwards to all segments except the source segment. If the destination is same as the source, frame is discarded.
Switches have built-in hardware chips solely designed to perform switching capabilities, therefore they are fast and come with many ports. Sometimes they are referred to as intelligent bridges or multiport bridges.
Different speed levels are supported. They can be 10 Mb/s, 100 Mb/s, 1 Gb/s or more.
Most common switching methods are:
1. Cut-through: Directly forward what the switch gets.
2. Store and forward: receive the full frame before retransmitting it.
OSI: Switches are on the data link layer (just above physical layer) that’s why they deal with frames instead of bits and filter them based on MAC addresses. Switches are known to be used for their filtering capabilities.
VLANs (Virtual LANs) and broadcast domains: Switches do not control broadcast domains by default, however, if a VLAN is configured in a switch it will has its own broadcast domain.
*VLAN is a logical group of network devices located on different LAN physical segments. However they are logically treated as if they were located on a single segment.
Bridges are used to extend networks by maintaining signals and traffic.
OSI: Bridges are on the data link layer so in principle they are capable to do what switches do like data filtering and separating the collision domain, but they are less advanced. They are known to be used to extend distance capabilities of networks.
In a comparison with switches, they are slower because they use software to perform switching. They do not control broadcast domains and usually come with less number of ports.
Routers are used to connect different LANs or a LAN with a WAN (e.g. the internet). Routers control both collision domains and broadcast domains. If the packet’s destination is on a different network, a router is used to pass it the right way, so without routers the internet could not functions.
Routers use NAT (Network Address Translation) in conjunction with IP Masquerading to provide the internet to multiple nodes in the LAN under a single IP address.
Now a day, routers come with hub or switch technology to connect computers directly.
OSI: Routers work on the network layer so they can filter data based on IP addresses. They have route tables to store network addresses and forward packets to the right port.
Gateways are very intelligent devices or else can be a computer running the appropriate software to connect and translate data between networks with different protocols or architecture, so their work is much more complex than a normal router. For instance, allowing communication between TCP/IP clients and IPX/SPX or AppleTalk.
OSI: Gateways operate at the network layer and above, but most of them at the application layer.
P.S. The term Gateway is used to refer to routers in some articles so beware. In this case, the router has gateway software. And Default Gateway is used to refer to the node (e.g. router) connecting the LAN to the outside (e.g. internet).
Repeaters are simple devices that work at the physical layer of the OSI. They regenerate signals (active hubs does that too).
There is an important rule to obey while using repeaters/hubs to extend a local network and is called the 5-4-3 rule or the IEEE way. The rule forces that in a single collision domain there shouldn’t be more than 5 segments, 4 repeaters between any two hosts in the network and only 3 of the segments can be populated (contain user connections).
This rule ensures that a signal sent over the network will reach every part of it within an acceptable length of time.
If the network is bigger, the collision domain can be divided into two parts or more using a switch or a bridge.
What have been introduced so far are the main traditional devices used to build networks, understanding how they work helps to understand the logic behind networks designing, however, now that technology advance quickly, it is possible to find new products in the market combining two or more of these devices into one.
- Brouter: Works as a Bridge and as a Router.
- IP Switch or MultiLayer Switch (MLS): New switches with routing capabilities, they forward data based on IP addresses, work at the network layer too.
References used are the following in addition to my previous readings and background study.
 TCP IP in 24 hours by SAMS