The modem, the router, and the gateway

Discussion in 'hardware' started by YeOldeStonecat, Oct 8, 2010.

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  1. YeOldeStonecat

    YeOldeStonecat Registered Member

    Apr 25, 2005
    Along the Shorelines somewhere in New England
    Rather than continue it as an off topic derailment in another thread, I started a thread dedicated to the subject, yes Bill this is for you. Yet to keep it civil, this is the "hardware" forum, a discussion about network equipment I don't see as breaking the rules. And hey, other people can learn about networking equipment.

    The disagreement is over the definition of Gateway, Bill likes to call a modem a "gateway", and I disagreed. Seeing that Bill is very literal and particular about facts, and likes to "educate" people about the meaning of "router" in that the "marketing weenies" twist around, and the fact that a wireless router is in fact 3x discrete devices in one box (which I agree to)...I also maintain educating people in the correct terminology.

    Bill, todays broadband modems are not too different from the dial up modems of yesteryear. They give your computer a public IP address from your ISP. Now I'm talking about pure bridged modems here..most often found with home cable broadband accounts. DSL broadband accounts used to use modems years ago, but most of them have switched over to providing the consumer with "combo" modem routers, in which case yes we can call them gateways. Why?

    Well, a pure bridged broadband modem simply passes the public IP address to the computer or device that is attached to it. The pure bridged modem itself does not stand in between the attached device with an IP address and route traffic, it passes traffic. If you look at the IPCONFIG output, you will not see an IP address for that device, you will see a public IP address that your computer obtained from it (or your routers WAN port), and you will see your default/remote gateway being what your ISP gives you on their network, whatever the nearest branch of their network is for you.

    A pure bridged modem (lets say..a Motorola Surfboard 5120) does not route traffic, it does not do any NAT, it does not take an IP address between your computer and the internet, it simply bridges your computer to your ISPs network, your computer obtains a public IP address from your ISP.

    Now the industry standard definition of a "gateway" is a network device that connects two networks/joins two networks/acts as an entrance to two networks.

    Now, the term "gateway" remains fairly loose, in that different devices, both hardware and software, can perform this duty...such as higher end devices like proxy servers and such. More commonly it is a router. But the important thing to remember, is the mandatory definition of "connect two networks".

    Now back to, a "modem" simply passes the ISPs leased IP address to the device attached to it. If your computer is plugged right into the modem, your computer picks up that public IP address that your ISP leased to you, just like it did back in the dial up days. Under this situation, the modem does not stand in between you and the ISP with an IP address, your computer is directly on your ISPs network, your computer is not on its own network, your computer is sitting on your ISPs network, because your computer picked up one of your ISPs IP addresses. Run an IPCONFIG /ALL. Notice the IP address, and most importantly...notice what's mentioned for a gateway. Yup..that gateway is something located waaaay up in your ISPs data center, connecting their network (which your computer is a direct member of ), to the internet. Thus completes the definition of gateway, two networks were connected.

    Now a router is a device that is commonly used as a gateway. It can perform other functions if you scale it up to enterprise situations, but...most commonly, and especially in smaller networks and home networks, it performs a similar task as a gateway....connecting two networks together.

    Home and SMB broadband routers run in "gateway mode" by default. You can see in many home grade routers, if you poke around the web admin, depending on your make and model and what it supports you may find a radio button/toggle switch to run in either gateway mode or router mode. The gateway mode is how it is run for the majority of home networks, the router mode allows it to be used as another function of the router, say in connecting branch offices of a wide area network. Different subject from this thread.

    Now if you plug your cable modem into your home broadband router, and you plug your computer into the back of your home broadband router, and if you run an IPCONFIG /ALL from your computer, you'll find that your computer is on what's called a private class C IP address, often something like Even if your computer is the only computer plugged into that device, the fact is..that computer is on its own network. That IP address belongs to your computer, not to the ISP. Going back to your IPCONFIG results...notice a gateway? That gateway is probably that is your home routers LAN IP address, your router has an IP address because its a network device in between two networks. Log into the routers web admin, go to the status section, and look at the WAN/Internet port..and its IP address. You'll see the public IP address that your ISP had previously given your computer, but now it's giving it to your routers WAN port. Your home broadband router sits in between your computer, and your ISPs modem. And guess what it's doing? It's "routing traffic between two networks"....and's "connecting two networks". It is the gateway between your computer and its little home network..and your ISPs network. ;)

    Now lets unplug your computer from the back of your home router, unplug the router from your cable modem, and plug your computer directly into the cable modem. Run an IPCONFIG /ALL again. Look at the IP address of your's back to the public IP address that you probably saw on the WAN port of your router when you looked at its web admin. Look at your computers gateway's back to your ISPs main router, or backbone to the internet. It's not the IP address of the modem, and it's no longer the IP address of your routers LAN port.

    Gateways connect two networks.
    Routers connect two networks, and in simple network terms they are the gateway
    Modems simply pass an IP address from their host (your ISP), to the device connected to them (your computer). That IP address is your ISPs network...thus whatever is connected to them is technically hanging on that same network..the ISPs network, with a public IP address.

    Now to add possible confusion to the list, many DSL ISPs provide little devices (like Motorola 2210 or Siemens Speedstream 4100 or 4200 models) that are actually combo modem/routers. By default many ISPs ship them with the little NAT router enabled, they do the PPPoE for you, they do NAT for you, they provide the device plugged into them (like your computer) with a private IP address like In this case, what you really have is just like you had your Linksys router plugged into a cable modem..except it's in one tiny box. Your computer is technically on its own little network, and since the device is running in router mode, it's the gateway, connecting two networks. Since this one tiny box is doing those 2 jobs...modem and router, it's commonly just called a "modem"..although that's technically incorrect. It is, in fact, 2x discrete network devices in one box..a modem and a router. Gee...that sounds familiar. "wireless fact 3x discrete devices in one box..a router, a switch, and a wireless access point/bridge".
  2. Bill_Bright

    Bill_Bright Registered Member

    Jun 29, 2007
    Nebraska, USA
    I am not going to argue your points for, truly, I might as well be talking to a wall - that sincerely is not meant to be inflammatory, but the reality is, YeOldeStonecat, you don't listen. For example, I have not denied that a router is not a gateway device. In fact, I have said a router is often the gateway device. But I also said repeatedly that in a home network, the cable or DSL modem serves as the "Internet gateway device" and I stand by that statement. I also agreed that on a basic level, a modem, as in a modulator/demodulator, is not a gateway in itself. But we are not talking about a basic modulator/demodulator. We are talking about home networks here folks, and we are talking specifically about a specific home network device, the cable or DSL modem.

    I am going to define the parameters of my comments, however. To start, I am referring to home networks. Not corporate networks, but home networks where the user subscribes through an ISP to receive broadband service, most commonly via cable or DSL. So IN THE CONTEXT of these discussions - that is, home networks, my comments are, and always have been in reference to specifically cable and DSL modems as found in home networks. I did not, nor do I make a generic claim suggesting that all modems are gateways, and I ask once again that you stop misquoting me, twisting my words around, or taking my comments out of context.

    In addition to not listening to or understanding my position, it appears as if you don't even pay attention to your own words. :doubt:
    That's a good definition of a gateway device, so thank you! Once again, you have validated my point! Note it does not stipulate that a gateway must be a router, or any specific network device. There is NOTHING, absolutely nothing in YeOldeStonecat's definition about passing IP addresses, bridging, routing traffic, NAT - NOTHING! To corroborate, and as I noted in that other thread, citing the The IT Library, Networking Glossary, the definition of "Gateway" is (my bold added),
    So by YeOldeStonecat's own definition, a gateway connects two networks. And in a home network, as we all know, you don't have to have a router! Millions and millions of home networks don't. All you need is "a device" to connect "two networks"; the home network and the Internet, and that device is the cable or DSL modem. So, as I have repeatedly stated, in that capacity, the cable or DSL modem acts as the Internet gateway device. If you have a router on your home network, you can remove the router and still connect to the Internet. But can you remove the cable or DSL modem and still connect to the Internet? NO! Therefore, the cable or DSL modem acts as the "Internet gateway device". That's not hard to see.

    Again, you validate my position and to further corroborate, and as I previously noted in that other thread, "gateway" is a generic term for a network device that can be implemented totally in hardware, totally in software, or a combination of hardware and software. It can be a router, it can be a computer. But for "residential gateways" the gateway device used most often to connect the "home" network to the Internet is the Cable or DSL "modem".

    So, one last time, and in conjunction with YeOldeStonecat's own definition of gateway, in home networks (even if a networks of one computer) it is the cable or DSL modem that "connects two networks" and therefore, it is the cable or DSL modem that acts as the "Internet gateway device".

    Now I am tired of discussing this. You provided enough evidence to validate and substantiate my claim. I provided more. You provided absolutely nothing that states the cable or DSL modem cannot be a gateway device. Nothing! The horse is not only dead, it is totally decomposed.
  3. Cudni

    Cudni Global Moderator

    May 24, 2009
    We all agree to disagree. Thank you
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