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I have currently access to Internet provided via cable modem, with single Ethernet output port. From earlier, I have PENTAGRAM Cerberus P 6331-4A ADSL modem/router. Would it be possible, and how, to configure said ADSL router to serve just as a hub / switch, for sharing my Internet connection. I'm especially interested in using the WiFi part of it.

PENTAGRAM Cerberus P 6331-4A has built-in 10/100 Mbps 4-port switch for LAN, 1 x WAN port for ADSL-type connection, and Wi-Fi 802.11b/g. I want to conect cable modem to one of Ethernet ports, desktop PC to other Ethernet port, and use WiFi connection from laptop.

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This might not be applicable to you (probably isn't), but it is technically possible to through software convert the WAN port to be just another LAN port, to get a 5 port switch. I sadly haven't seen this option with native firmware, but with e.g. dd-wrt and the likes it is often available (I guess your router does not work with dd-wrt (I don't recognize its name, at least), but it can be something to look for anyway - it should probably be a configuration option named "Use WAN as LAN port", "Convert WAN port to layer 2" or similar). –  Daniel Andersson Apr 24 '12 at 14:16
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4 Answers

If your ISP can send multiple IPs through the circuit and you get a cheap switch, then you'd pull IPs in parallel by static assignment, keep in mind there is no firewall and your IPs are 100% public with this option. If you turn off DHCP and NAT, then you should be able to use that ADSL modem/router as the switch for this scenario.

If your ISP is only sending you one IP and you can't get more than you must use DHCP/NAT, hence you must use a router.

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On many "home" routers the 4 LAN ports and the Wi-Fi "port" are just switched and not routed. So just connect the ethernet cable from the cable modem to one of the four LAN ports and you should be ready to go (provided that 1. the cable modem has a builtin DHCP server, and the DHCP server is enabled, 2. the router and cable modem are on the same subnet and 3. all clients are set to request an IP via DHCP or they're otherwise manually set on the same subnet).

At home I have a dlink gateway (adsl modem+router+wi-fi ap) that I use exactly like this.

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All routers have only one LAN port, the multi-port LAN portion is always switched. There is no other way, it just happens that they are connected internally on all these "home" devices, and even on enterprise ones. There is no such thing as a router with multiple LAN ports, they must be switched or set on a VLAN. –  MaQleod Nov 10 '10 at 20:50
    
one drawback to this method is that you will not be able to take advantage of any of the security features that plugging into the WAN/Internet port of a stand alone router would provide –  Xantec Nov 10 '10 at 20:53
    
I would have to configure ADSL because it uses different IP subnet (192.168.1.*) than cable modem (86.sth.*.*, if I remember it correctly). –  Jakub Narębski Nov 10 '10 at 21:23
    
if it starts with 86, then you can't change it. A NAT address will start with 192.168, what you have is a public IP that your ISP controls. If that is the case, you need to look at my answer. –  MaQleod Nov 10 '10 at 21:48
    
If it is a modem/_ROUTER_ as you wrote in your question then the 86... must be the external IP and you should be able to configure the internal IP as you see fit. –  CAFxX Dec 2 '10 at 11:35
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It's very simple ..just disable your DHCP in the ADSL router and if u have shared your cable internet using Internet Sharing connection then the Internet Sharing connection will provide the IP address for you and enjoy your ADSL router as wi-fi device

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when I look at this question to me like you got a modem/router from one isp, have since moved to another and their modem doesn't do any the fancy stuff the other one did, and since your switching from dsl to cable there's no way to use the original modem as a modem, but you still want to share your connection and use the wi-fi off the original modem. and while it's possible to plug in the cable modem and other computers in. and configure the old modem to act as a switch/AP instead of a dsl modem, it can't do NAT in that state; which is the key component of connection sharing. and I'm guessing the reason your asking about needing connection sharing is because the cable modem does not do that. one way you could find out if the cable modem is actually doing NAT and thus is ready to share the connection once there's a way for more then one computer to be physically connected to it, or not which then would require some other device to sit between it and the computers to do the NAT for the connection sharing to work. would be to check what Ip address your getting if it starts with 10. or 192.168. when your connected directly to the cable modem (without the dsl modem) then yes it is, and it won't be a huge issue to use the DSL modem; you just need to set the dsl modem to use the an ip on the same subnet as your computer got from the modem and turn off the dhcp on the modem and then it's ready to go as a eithernet switch and Wireless Access Point. but if you get an IP that starts with some other set of numbers then the two I got listed then that means to share the connection you need some device to do NAT that would sit between the two modems. and unless you have a wired router sitting around then it's going to cost just as much for a wired router as a wireless one and be a whole lot less hassle to yourself a wireless one. (if you do got a spare working wired only router, then use that to set up a shared wired connection between the cable modem and the router. then you can follow the same steps as if the modem was doing NAT, just remember to plug the reconfigured dsl modem into the wired router, not the modem)

if your wondering what NAT is, it stands for Network Address Translation. and it is the mechanism that allows multiple computers to share one "public" ip address, as it translates the requests from the private ip's your computers gets to and from the one public ip you get from your isp. an analogy here would be sending postal mail out to addresses, where there is not room on the label for an apartment or suite number. people who live in a house are unaffected by this as each has a unique street address (aka only one computer on the connection), but people in apartments will not be so lucky, as they all have the same street address as all the apartments are in the same building,(aka multiple computers on one connection). so the mail carrier has no choice but to drop off all the mail at the front office as that's all they have is the street address to go on. NAT would be like the person in the office who examines the letters to determine which apartment the letter was intended for and sorts out the mail for each apartment. it sorts out which packets are meant for what computer so the can get routed back to the computer that was requesting the data.

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