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Q: Connecting a router to a hub ( Answered,   0 Comments )
Subject: Connecting a router to a hub
Category: Computers > Internet
Asked by: rgm-ga
List Price: $20.00
Posted: 30 Jun 2003 15:37 PDT
Expires: 30 Jul 2003 15:37 PDT
Question ID: 223700
I have a cable service with three ips. I have 6 pc's connected through
a router. I want the router to have an ip for 5 pc's and one pc to
have an ip all to itself. I conclude the solution is to connect a hub
to the cable modem and the router and one pc to the hub. Is this
correct? And if so, how is it done.
Subject: Re: Connecting a router to a hub
Answered By: haversian-ga on 06 Jul 2003 03:14 PDT
Good morning rgm-ga,

Yes, you've pretty much got it.

The specific configuration details will depend on what hub, router,
and PCs you are using, but the general principle is the same in any
case, and should be enough to get you on your way.

Your ISP provides IP numbers through DHCP (Dynamic Host Configuration
Protocol), which allows a computer to request an IP number and to be
given one not currently in use.  How your ISP decides what number(s)
you will get would depend on their configuration, but the basic idea
is that you plug a machine in and it works, so the details probably
will not affect your procedure.

Plugging the cable modem into a hub or switch will allow multiple
machines to access the DHCP server and request their own IP numbers. 
One of these machines will actually be your router.  A router has two
parts, an upstream and a downstream.  The upstream half will request
an IP number from your cable provider's DHCP servers just as a normal
PC would.  The downstream half provides its own DHCP service to the
computers you plug into it.  These computers will get special IP
numbers known as non-routable addresses because they are reserved for
private networks.  In effect, the router acts just like your ISP,
providing IP numbers and routing packets.  Whenever one of the
machines connected to the router tries to go somewhere on the
internet, the router accepts the request and relays it through its
upstream half to your cable provider, which sends it on its way.

If you do not already have the hardware, you can probably find a cable
modem / router combination device which will provide a port in what is
typically called a 'DMZ', meaning it is not hidden behind the router. 
This would give you in a single box the functionality you require.

Please don't hesitate to request additional information if necessary. 
These sorts of "How do I?" questions sometimes take a bit of
communication back and forth to get all the details ironed out.


Request for Answer Clarification by rgm-ga on 06 Jul 2003 06:30 PDT
Thank you for a very clear and concise explanation about DHCP and IP
assignments. I have been trying to get the connections to work and I
have failed. I have a 3Com Hub and a Netgear router. I need to know
what ethernet cable to use between the modem and the hub (regular or
crossover) whether to connect the modem to a regular hub port or to
the uplink port and then whether to connect the router modem port to a
regular hub port with a regular cable or crossover cable. I think I
have tried all possible combinations but I have not been able to get
the router to work. I must be missing something really simple. The ISP
assigns ips automatically and all pcs work through the hub without
problem and through the router without problem.

Clarification of Answer by haversian-ga on 07 Jul 2003 23:15 PDT
> I need to know what ethernet cable to use between the modem and the

To be honest, I've never done it that way.  I've always used an old
computer running BSD or something to act as a firewall.  Then, the
modem is plugged into the computer normally, and BSD provides name
resolution, DHCP services, filtering, firewalling, traffic shaping,
and all the other network goodies I need.  Let me get back to you on
that one - I've got a 3com switch and cable modem here to run some
experiments.  I strongly suspect that you will use a regular
straight-through cable though, as the hub will provide any "crossing"
it needs.

> whether to connect the modem to a regular hub port or to the uplink

Well, when you've got a firewall in there, you use a regular port - my
network doesn't use the uplink port for anything.  I'll get back to
you on this one too with something a bit more concrete than the usual
conflicting reports one finds on the net.  I can't think of any reason
you would need to use an uplink port on the hub.  You will need to be
using the right port on the router though.  The same port that you
normally would plug the cable modem into.

> The ISP assigns ips automatically and all pcs work through the hub
problem and through the router without problem.

I'm not sure I understand that part.  Do you mean that the machines
can see each other through the hub, with no modem installed?  And
similarly that the machines can see each other through the router,
with no modem installed?  What happens when you plug one machine into
the hub, one into the router, and the router into the hub?  Does that
break it?

Could you find the exact model number on the router for me?  The hub
should be generic enough that it won't matter, but how the router
operates could be important.

My gut feeling on this is that quite a few configurations will work; I
suspect the problem is one of configuring the router, rather than
cabling, though of course you have to hit on one of the right
configurations for the cables or the rest of it doesn't matter.

Clarification of Answer by haversian-ga on 10 Jul 2003 16:10 PDT
Just letting you know I haven't forgotten about you.  Early tests
reveal "it doesn't seem to work" but that was on a switch that has
decided to stop working and has been replaced, so I'll re-run my
experiments, probably Monday.  Sorry for the delay.

Request for Answer Clarification by rgm-ga on 12 Jul 2003 05:27 PDT
The hub is a standard 3Com hub and the Router is a Netgear. Thank you
for your interest.


Clarification of Answer by haversian-ga on 15 Jul 2003 19:19 PDT
I think we should do this in stages, because I have a feeling if we
try it all at once, it won't work, so this way we'll have a better
idea *what* exactly isn't working.

Ok.  Plug the cable modem into one of your computers (using the cable
it came with).  Set the computer to use DHCP.  That works, I presume?

Plug the cable modem into the 3Com switch (using a straight-through
cable), and plug the computer from part 1 into the switch (also using
a straight-through cable) as well.  Still set to use DHCP, does that

If that works, plug two computers into the 3Com switch and set them to
use DHCP.  Does it still work?

Unplug one of the computers and plug the router in instead.  Plug the
computer into the router using a straight-through cable.

Plug two computers into the router using straight-through cables.

As I said, I expect somewhere along the line that won't work (it's
been uneven for me whether it works or not, depending on how I have
things configured).  Let me know where along there it fails and we'll
try to figure out what's up.


Request for Answer Clarification by rgm-ga on 16 Jul 2003 15:00 PDT
It is very impractical for me to do what you ask because of fisical
location restrictions. I would need for you to give me a final
guestimate...whould you in my place spend more time trying to find a
solution as to how to connect the router to the hub...or would you
give up because it is likely that it cannot be done. Thank you for
your interest.

Roberto Mestre

Clarification of Answer by haversian-ga on 24 Jul 2003 10:43 PDT
Hello again.

I only mention that I doubt it will work because there is always some
little detail that turns out to be important, that is different.  The
progression I gave should work, but I split it out into phases like
that, so that if (when) something goes wrong, the point of failure
should give us the most possible information.  As I understand it, you
tried what seemed reasonable, and it failed.  This way, we will know
more accurately what failed, and can try to address that problem.

Given that there is no reason that I can think of that it wouldn't
work (subject to certain assumptions about how your router works, and
how your ISP is assigning IP numbers), I would continue to pursue the
problem.  I don't know how important to you the solution is though, so
you will have to be the final judge of whether to proceed or not.

Certainly there are other ways to accomplish what you're looking for,
and I don't know what the relative importance of a fast solution
versus a cheap solution is.  This way should be the cheapest, as you
already have all the hardware you should need, but it is turning out
to be a slow one, particularly with your distance from the location in

Let me know how, and whether, you want to move forward on this.



Request for Answer Clarification by rgm-ga on 19 Aug 2003 16:15 PDT
Just returned from a very long trip. Will re read your recommendations
carefully. Thank you for your interest. If I get stuck I'll let you


Clarification of Answer by haversian-ga on 26 Aug 2003 18:48 PDT

(just need to clear the "needs clarification" bit so if you *do* need
more help, it will be noticed!)
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