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Modem connection works, Router connection doesn't

Posted on 2012-03-11
Medium Priority
Last Modified: 2012-03-15

I am using Linksys router WRT54GL. My router is connected with modem and everything works fine - I can connect computers with UTP cables via router / modem. I can also have wi-fi from linksys router. All OK.

I only have one problem. House is wired with UTP cables, so I simply plug in the cable in the upper floor in the switch in the wall and I have internet. And here's the problem - If I connect internet through modem, all is fine (so, plug in cable from modem to the switch in the wall). If I do the same thing from the router - I don't have an internet connection.

I connect router through PPPoe with user name / password.

What is the difference between router and modem connection?

To be more clear: If I connect computer directly to the router with cables, it works. Only if I do this by first connecting router with the network already installed in the house, then it doesn't work - computers in upper floors don't have internet connection. Doing same thing with modem - all OK.

And idea?

Regards, Frenky
Question by:AntonioRodrigo
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Expert Comment

ID: 37708645
Not quite sure your decription if you can draw a floor plan and equiomemt instataion it is better.

Make sure router WRT54GL 's WAN port connect to the modem and your PC/notebook plugin the other ports.

any signal led lamp "ON" at the port ? i think u may need a cross cables if there is no signal.

If directly connect to modem is work but router is not,.

Author Comment

ID: 37708668
Thanks for response, Barry.

A little more description: internet connection comes to the first floor (via telephone cable). There I have a splitter, modem and router. House is wired with UTP cables, so if I plug in cable from modem to the wall (first floor), other floors also have internet. The problem is, if I plug cable from router to the wall - then upper floors don't have internet.

But, if I connect computers directly to the router, I have internet. This is strange to me - what is the difference between plug in modem or router in the house network? Why is modem giving me the internet connection and router doesn't?

Expert Comment

ID: 37708917
Can I please ask what your modem is? - it sounds like it MAY be a router modem, and you have the LAN connection connected to the UTP in the house. If so, every machine that would connect to the router modem (your ORIGINAL equipment which connects to the internet) would receive an IP address and would consequently get an internet connection.

If you THEN connect the WRT54GL to the wall in the upper floors you MUST ensure that the WRT54 is set up to provide IP addresses (ie it is acting as a DHCP server) and that the output from the wall is going into the WAN port on the router.

May I ask what is the reason you are using the WRT54GL - is it to get an otherwise unavailable wireless connection?

You stated in your original question that you are connecting the "ROUTER through PPPoe with user name / password." If you are only using the ROUTER (ie the WRT54GL ) to distribute the signal around the property, this should NOT be necessary.

As Barry said, a diagram would be REALLY useful....
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Author Comment

ID: 37709004
Only one thing is not clear to me - if I connect WAN port from the router to the wall, how will I connect modem and router? I have UTP cable plugged in router from modem via WAN port, and from the router to the wall via one of the ports on the router.

I agree that connecting through PPPoE is not necessary - I will try to make connection via automatic DHCP (on Linksys this is the first option).

My modem is also the router - it has wireless antena on it. It is the modem my ISP gave it to me.

I am using WRT54GL because it has stronger signal than original modem/router. WRT54GL has two antennas and it is capable of providing wireless internet on the upper floors.

I am sorry, but I don't have a diagram. The network in the house was not built by me. Siimply floors are connected through walls (so, cables are wired inside walls). That's basically all I know - and, of course, that it is working if I plug in modem into wall in the first floor.

Expert Comment

ID: 37709043
Nearly there Frenky!

Why, if you already have wireless and multiple LAN sockets on your original router, are you choosing to also use the WRT54?

My understanding of your useage was that you had the router (original modem) on the first floor, and were trying to connect the WRT54 on ANOTHER floor - is this not correct?

If you wish to distribute the wired connection among multiple floors you have obviously sorted out that you simply need the original modem connected to the wall - where does the WRT54 come in?

If you are connecting the modem AND the WRT54 on the same floor, it would be necessary to connect the cabled output from the MODEM to the WAN port of the WRT54, then one of the LAN ports to the wall to distribute the wired signal around the house. I cant see any practical benefit to that, however.

If you wanted to use the WRT54 on another floor you would leave the original modem connected to the wall and connect the WRT54 to a wall socket on another floor via its WAN port. This would then effectively give you wired and wireless disctribution of the ORIGINAL signal (from the wall) VIA the WRT54 (assuming you are using DHCP on the WRT54)

Author Comment

ID: 37709067
Let me explain a little bit further: this all thing is not for me, it's for my client. He has modem and router - WRT54GL router because of the strong wireless signal. He has lost connectivity throguh router after two electricity power losses -> he said to me that everything was working fine through router - even internet in the upper floors. I've found it out that router lost all settings, I guess it was because of power losses. So I've reconnected router to internet via PPPoE.

I've also found it correct to simply connect modem and with upper floors (that's what I did and it works and it even doesn't bother him). My question was because it didn't made clear to me that connection in the upper floors is working via modem and via router doesn't (at least that's what he claims).

Chris yours answer about DHCP seems very clear to me, good point. Before that I didn't even know why DHCP is used for. Although one thing still remains mystery: why I can connect computers via cables on my router, but I can't connect computer on the upper floor through router / wall? If router is connected via PPPoE, how can it give four IP's in it's ports?

Accepted Solution

chrisalis earned 2000 total points
ID: 37709676

If your modem is a standard adsl modem, it can normally supply a full range of IP addresses on an internal non routable range (typically set up to provide something like - ); it ISNT NORMALLY (and this is where I think you may be getting a little confused) providing ROUTABLE IP addresses (something like which have access from the internet

The process of Network Address Translation (or NAT) that goes on in a router is a little bit out of the scope of this question, and would take me about an hour to explain - suffice it to say that from the one PUBLIC address that your modem gets from the ISP, it can distribute that signal to a number of PRIVATE addresses on a network that it manages and provides private addresses for.

Your final question about not being able to connect upstairs computers may have a number of answers. The first thing to do would be to check that the upstairs computers are actually getting an IP address from the WRT54GL - they MAY have had their addresses "fixed", and having reset the WRT54GL, they may now be in the wrong range.

Here's a thought - this would resolve the problem and leave all parties happy;

Take ONE CAT 5 cable from the original modem/router and plug it between the wall and the modem; this will give you the upstairs connection. Then take a SECOND Cat 5 cable from the SAME router and connect THAT to the WAN port of the WRT54GL; THIS would give the strong signal that the client wants, but without the wired signal having to go throughthe WRT54GL to the upstairs machine.

Author Closing Comment

ID: 37709709
This is definitely very very well explained and it solved my problem.

Author Comment

ID: 37709750
I immediately checked IP of three computers connected via Linksys router - if I type ipconfig, their IPV4 addresses vary. But, if I try to find IP address that those computers use to communicate with the outside world, they all have the same address. I am doing this through website http://whatismyipaddress.com/.

I guess the difference in private / public addresses is because of NAT ?

Expert Comment

ID: 37709799
Spot on Frenky!

If you get a chance, try looking up NAT - a understanding of it is essential to a PC engineer; along with DHCP and TCP/IP its "must know" stuff for diagnosing in the field.

Heres a basic site which will help out on NAT -

http://www.homenethelp.com/web/explain/about-NAT.asp - the info is over 10 years old (ancient by PC standards) but still highly relevant

Good luck!

Author Comment

ID: 37718563
Very good article, thanks ))

Chris, if you are still around - I was watching this morning configuration in one company. Modem, connected to router and router to switch with 17 ports.

What seems interesting for me is that two printers are connected directly to router. To router is also connected switch. All the computers (seven of them) are connected to switch.

So switch is simply NAT provider? Is there a difference if computer is connected via router or via switch? I guess modem could also be connected to switch directly and all the computers and printers also?

Expert Comment

ID: 37718855
Hi Frenky - still here - I enjoy the follow ups.

Nice to see you getting to grips with this - lets go through your stuff.

The switch is probably NOT providing NAT in this scenarion, its almost certainly just redirecting traffic to the correct machines, What you have in the Linksys router is a small four port switch, with wireless network and NAT (possibly print servers as well - see below) - larger ones are just used to extend the size of the network (number of pieces of equipment attached) - they may do OTHER jobs, dependant on their complexity, but mostly, thats it.

The router that you mentioned (similar to your linksys), is either a simple access point, which provides wireless to the network, or more likely is what I refer to as a CABLE router, with a WAN and four LAN ports; this DOES provide Network Address Translation, where one network (the internet, for example) is connected to another network with a different set of IP addresses. It may also act as a PRINT SERVER, where you can connect a printer or printers to special USB sockets on the router and those printers may then be addresses by any machine on the network. More normally, however, those printers will be network printers, with the facility to connect direct to a network (as opposed to being shared off a machine)

The most likely reason for connecting the printers direct to the router is that the router is acting as a DHCP server - in other words, it is allocating local network addresses to all machines on the internal network. Its probable that the person setting up the netwoirk has put the printers on the router to minimise traffic through the switch- but that CAN cause a network bottleneck as everyone tries to access the printers via the (single) connection between the router and the switch; if I were setting it up, I would probably do it with the printers in the switch.

And finally, YES - the modem COUld be connected direct to the switch; as long as the modem is acting as the only DHCP server, allocating the addresses to the machines on the network this should work. There may be other reasons why this has not been done, however - individual circumstances need to be taken into account. We have NOT for example, considede the role that a SERVER may play in the above scenario - but thats a game changer!

Good luck in your investigations.

Author Comment

ID: 37723767
Chris thanks again for very good answer. This NAT and DHCP thing are still not completely clear to me - how I understand NAT: from one IP (let's say, IP from modem to router) it gives local IP's to all the computers connected to that router? Router and switch can do this, I guess. I have connected modem to router and router to switch.

I've done a little experiment: I reconnected my computer from router to switch. The IP was the same and it was local IP, like Then I connected my computer with modem - the IP was the IP that my computer uses to connect to the outside world.

So it seems that modem has several IP's that can give through it's ports - but router simply gives local IP's to everyone connecting to it?

When I connect modem and computer, the IP was not the same as on the other computers (I was looking at the IP with www.whatismyipaddress.com)

But, when connected to router / switch that IP was the same for all computers...

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