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.
Examples are:
- 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.
