Networking things Was: RE: [Techtalk] DHCPD Question

Kath kath at
Thu Oct 11 19:51:47 EST 2001

Not exactly on those definitions, but close :)

Definitions of a hub, switch and router, according to Cisco stuff (Not
actually what it is, as I'll show)

A hub works at Layer-1, the physical layer.

A hub works on simple electricity.  Stuff in, stuff out.  It is also called
a 'repeater'.  It is dumb.

They usually have just Ethernet ports.

A switch is a smarter hub; it does all the hub stuff and more.

A switch (by definition) works on Layer-2, the datalink layer.

On every network card there is a MAC address, which uniquely identifies the
computer.  The switch will do a broadcast and poll the devices on each port
for a list of MAC addresses of connected devices.

When traffic goes through the switch, the switch looks at the MAC address on
the packet of the source and destination.  Here are what it could do with
the packet:
- If the source and destination are inside the same port, it will drop it).

- If the source is on one port and the destination another, it will forward
to that port.

- If it cannot find the port in any table, it will either broadcast it out
to every port or sometimes broadcast out to a specified port.

This can be used to effectively control network traffic to an extent.  It is
also more secure than a hub, as a hub repeats all information in on one port
out to all other ports.  It however will not stop multicasts, because
multicasts are IP broadcasts and MAC addresses don't matter.  However, newer
switches can control them.

A bridge is a nonlonger used variety of a switch, except the bridge only has
2 ports.

A switch will have usually have Ethernet, fiber and if it is configurable, a
console port.

A router is a switch on another level.

A router works on IP addresses, which is Layer-3.

It works more the same, just replace MAC with switches with IP address.  A
router can also be configured to route all IPs to a certain path.

A router can have Ethernet, AUI, serial, fiber, console and etc ports.

Now, the industry seems to be merging to at least make the definition
between switch and router blur.

I have a feeling switches will totally replace hubs in a few years (This has
happened on the cable/DSL router level) and for routers and switches to
merge into one device, which will probably also include a stateful firewall

What you stated about speeds is a misnomer, you could easily have a hub that
only works at 10 or only 100 (I have one of each of these) or a switch with
the same issue of only one speed.  It is just really that when people pay
for a switch, 10/100 autosensing is included because of the price.  But now
I see devices going towards 10/100 for all.

Questions?  Feel free to drop them my way or put them out to the list. :)

- k

-----Original Message-----
From: techtalk-admin at
[mailto:techtalk-admin at]On Behalf Of Kai MacTane
Sent: Thursday, October 11, 2001 6:16 PM
To: techtalk at
Subject: Re: [Techtalk] DHCPD Question

At 10/11/01 03:15 PM , Raven, corporate courtesan wrote:
>         At the same time, most modern routers are also capable of doing
>port-filtering and layer 4 stuff, and I have heard rumors of stateful
>firewalling coming soon.  Kinda makes it difficult to be a beginning
>networking student -- they tell you routers are layer three devices, and
>it's a big lie.  [grin]  1, 2, 3, and usually 4.

Wow, no wonder I can never quite seem to figure out the difference between
all those different kinds of networking devices. My level of understanding
is something like:

    Hub: One of those things I have a pair of, sitting in my kitchen.
Switch: Like a hub, but can go at two speeds. (May only have two ports.
         (One of my hubs is also a switch.)
Router: A thing I don't understand, don't own, and don't need.

Which is pretty basic, I realize. (Luckily, I don't make my living
understanding network equipment.)

But at least now, I don't feel so *bad* about not understanding it!

                                                 --Kai MacTane
"When nothing's sacred any more,
  When the demon's knocking on your door,
  You're still staring down at the floor."
                                                 --The Chameleons UK,
                                                  "Swamp Thing"

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