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Conventionally, when we register for a broadband Internet connection, the ISP provides us with a Modem/router. The modem will have a port for phone-line connection, and (say)4 Ethernet ports. I was just wondering about the internal connections within the router. Which of the following is the correct internal arrangement?

  1. Modem to Router(implicit) to Switch(all 4 slots)
  2. Modem to Router(all 4 slots)
  3. Modem to Switch(all 4 slots)

Now, consider another scenario where we get a modem with only one Ethernet port for output. Now if I need to connect 20 PCs to it in order to connect to the Internet, will buying a (say)30-Slot Switch suffice (or) should I buy a router and then connect it to a switch?

P.S. I asked the two questions together because when I asked the first question to a senior, he answered quickly. Then when I asked the second question immediately, he stumbled and told that he was not sure.

P.P.S. My understanding is that the modem which comes with more than one Ethernet output slot, would likely have a router setup internally. And the modem which comes with only one Ethernet slot would not have an internal router, and so it is devoid of any routing capabilities. Am I correct?

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3 Answers

up vote 2 down vote accepted

Devices commonly handed out by ISPs are called modem/routers as they have both a modem AND router built in. In virtual blocks it would be Modem <=> Router <=> Ethernet slots. (<=> denoting traffic possible in both directions.)

In physical hardware, it may be contain in a single chip or broken up into separate devices.

For a router, all data comes into the router from any node, is sorted and sent out on the destination port that the router deems to be the fastest path for the packet. This may be out to the internet or to another node within the network. A router is also a gateway between the internet and a LAN. It carries its own IP address that is usually the gateway IP.

A switch (not including managed switches) are dumb. They are only interested in what the IP address of each node connected to it is and where the gateway is. If a packet comes in and the destination IP address is that of a node connected to it, it will simply pass the information to the appropriate port. If it doesn't know the IP address, then it passes it to the gateway and lets the gateway deal with it. A switch doesn't have an IP address.

Most modems these days are capable of acting as a gateway in a very limited capacity. But typically, a modems job is to do just that (MOdulate signals going out, DEModulate signals coming in).

For your scenario, if you were using a modem/router, you could plug a switch into any of the ports and you'd be on your way. If it was a stand-alone modem, you would need to install a router and then a switch.

Regarding your P.P.S. as I said above, most later model modems have limited capabilities such as DHCP servers and could theoretically be used with just a switch but if you're talking about networking 20 nodes, I would recommend setting up a router to take advantage of router features like port forwarding, firewalls and access control.

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What is the correct internal arrangement of a modem?

A typical modem will be option 1. The modem will be connected to a router, and the router will then be connected to a switch. The real nitty gritty gets much more complicated that that. For example, some of the Linksys 54G routers, use only a switch with vlan tagging to separate the WAN from the LAN ports.

Connecting 20 PCs to a modem

The modem likely has a built in router, however it is most likely only intended for a small office. It would be much better to connect the modem to a dedicated router, and then that router to a 24 or 48 port switch.

Some great options for a router for a small setup such as yours:

pfsense
clearOS
untangle

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The answer to your questions is (in general)

An ADSL router with 4 ports (they are routers, not modems), typically contain an ADSL port which is routed to the 4 ethernet ports. The 4 ethernet ports are typically individually accessible but bridged to behave like a dumb switch in software. [ I know this because I have reconfigured a number of these devices to have different switches on different VLANS and route between them by changing the software to OpenWRT ].

A single port ADSL router (Its still a router even if its called a modem) routes packets between the ethernet and ADSL interface, but is still routing the packets. We know this because the device will typically strip off the PPP overhead (ie the PPP bit in PPPoE or PPPoA) and then make that available to ethernet side of the connection. The devices also often run a DHCP server so they can hand out an IP address. These devices are generally slightly cut down version of 4 port routers.

An ADSL modem is the bit in the router which actually captures the raw frames. In earlier times, ADSL routers were internal cards (which could go into a PC) which did this - they did not have routing functionality on board. Remember too that a "modem" (or "conventional modem" is actually a device used to modulate a digital signal over an analog carrier - typically associated with speeds of 300 to 56400 bits per second and used over the phoneline in days gone by).

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Not sure why my answer was downvoted and I'd like to know if anyone can tell me !!! –  davidgo Aug 2 '13 at 23:30
    
+1 by Excellent Answer. –  DBX8 Aug 7 '13 at 1:56
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