2 cable modems, 1 lan?

We have 2 cable modems to connect to the internet. Each one of them can only connect 8 pcs (via a hub). There are 16 pcs and two eight port hubs (+uplink). Now there are two lans of 8 pcs, each one using one modem. We 'd like to connect both lans. How can we do that?
We 've got some extra ethernet adaptors and a five port hub if needed.

Thanks in advance

 

Hi Pros!t and welcome.

It sounds like you have an IP for each user if you are connecting a hub to a cable modem and multiple users can use the internet. If that is wrong, correct me.

There are several approaches to doing what you want but we need a little more information and you may have to make some changes.

Please give the IP address & subnet mask for any 2 PCs on each cable modem.

 

We can get 8 ips/cable modem.
ips are 10.4.x.y (x and y can be anything)
subnet mask 255.255.0.0

Someone said perhaps a bridge would do the job. Won't the cable modems 'fight'?
We can contact eachother with those local ips, but since we need to use the cable modems, the speed and the number of bytes are limited.

 

Excellent news about the IPs. Problem solved. And with the setup outlined below, the cable modems won't even really notice each other.

For this explanation, lets say you have

- 2 cable modems, Mod-A and Mod-B with the IP for Mod-A 10.4.0.1 and the IP for Mod-B 10.4.1.1

- 2 hubs, Hub-A and Hub-B and both are loaded with no open ports.

- 2 groups of PCs, Gru-A and Gru-B.

Take another Hub-C with at least 4 ports. Remove a PC from Hub-A and one from Hub-b and use crossover cables to connect Hub-A and Hub-B to any ports on Hub-C. Plug the PCs in to the open ports.

Any PCs that are to be in Gru-A and use Mod-A will need their default gateway address set to the address of Mod-A. It does not matter which hub they are plugged in to. The gateway address will define which modem they use for internet access.

Likewise with Gru-B PCs which will need their gateway set to the IP of Mod-B.

The PCs will be on the same internal network if you
- put them all in the same workgroup
- give them each a different computer name
- set up Client for Microsoft Networking on each PC

Performance note: You will not have particularly good internal communication speed between the PCs if you continue to use hubs. Several reasons for this.

• Hubs operate at 10Mbps half-duplex (meaning they are either sending or receiving but not both at once).
• Hubs share the available bandwidth so with 10Mbps to use and 16 devices, if all are busy at once you could be looking at around 1Mbps top speed for each PC.
• Hubs are stupid. The PCs will all need to communicate using broadcast messages so all 16 will need to pay attention if PC-1 tries to open a session with PC-12.

I strongly recommend you get switches to replace the hubs. You can use the same configuration with 3 switches and the crossover connection if you want or else get a pair of 10 port switches. No matter. The advantages will be:

• Switches will operate at 100Mbps full-duplex (so already a 20 times increase in speed).
• Switches can provide the full 100Mbps speed to all devices plugged in to them so each PC can run at 100Mbps.
• Switches have some logic available that hubs do not and will 'remember' where specific PCs are located so that if PC-1 wants to talk to PC-12, the switch will direct the packets and only PC-12 and the switches will notice.

Configuration note: If you got a router/switch for your network, you could get rid of one cable modem and only have to pay for a single IP address with the other one. The IP addresses you have are Private ones so you aren't getting a 'real' internet IP address now.

A router/switch like the Linksys BEFSX41 will run you around $70 US and can easily manage up to 253 users for internal networking and internet use. It has a hardware firewall built in to give you some additional protection. The BEFSR41 is slightly less expensive at around $50 but lacks the firewall. Thge cable modem would plug into the router/switch.

Both are 4 port switches so you'd still need additional switches/hubs to plug the PCs into but if you plugged your three existing hubs into the switch and spread the PCs out evenly, performance would be lots better than with the all-hub situation. Each hub would only have to deal with the devices plugged directly into it.

 

I didn't know those modems have a ip.
How do I find out what these ips are?

It's very usefull information, thanks!

 

Check the default gateway settings on a PC from each modem. Those are the addresses you will need to use.

You may need to do start~run~winipcfg and then click the button for details to find the gateway address.

I was assuming the modem had the address but it may not. No matter since you don't care where the gateway is; only that the PCs can find the proper one.

 

Houston, we have a problem.

The gateways are the same for all pcs (10.4.0.1). So if I leave it like that, both modems will be used.
Other suggestions to make each pc choose only one modem?

I tryed to set up a bridge, everithing is very slow then, and full of errors. I think it's the same problem. Can't I make a bridge that only works for the ips in the LAN? It should only forward the packets if the two pcs are on a different part of the network. (it should work as a switch, not as a hub )
The standard tool for bridging in WinXP is not good enough I think.

 

The subnet-mask has been changed to 255.255.240.0
I don't know if it helps, does it?

The winipcfg doesn't work, but I used ipconfig in the command prompt.

 

Sorry for the delay. Was fishing all weekend.

I'm going to have to think this one thru a little more. I'm missing something critical and need to figure out what.

As to the subnet mask, 255.255.240.0 is certainly easier for the PCs to deal with and should speed things up if your IP addresses are close enough together. It greatly reduces the size network you are dealing with.

 

Pros!t,

I dont know where you are located and whats avaible but with 16 users it sounds lie it's time to loose the cable modems and get a T1

Or, atleast a router and one cable modem so you have your own secure network.
There may be a way to bridge two connections and do some traffic shaping but not for less than 1000 dollars.

 

I'm located in Leuven, a city in Belgium. We are all students. The university is our isp. Because our residence is too far from the faculty, we're not directly connected to the university's network, but via a commercial cable provider. T1 is no option neither 1000 dollars.

If I ever find a way to solve the problem, I will post it here. Thanks for your help!