WAN Port on Cable/DSL Router

[wifiwhiz]

I'm confused. Can anyone tell me exactly what is coming out of (and going into) the WAN port of your standard DSL/Cable router? Intuitively I think of this as just standard 802.3 Ethernet exactly like what goes into and comes out of any standard ethernet NIC. Exactly what about this port or the associated electronics inside of the router makes it "DSL or Cable"?

For that matter . . . what about the LAN port on the back of your standard DSL modem (e.g. Efficient Networks DSL modem) or cable modem (e.g. Motorola Surfboard Cable modem)? Isn't this just an ethernet interface? Can't I just connect this to any 802.3 type device. I know I can connect my modem directly to a LAN switch (not a good idea, but can be done -- right?).

I have been having problems sorting out this technology in my mind because I am determined to understand what makes the WAN port so special on a DSL/Cable router. This port is often called the "Internet" port -- but isn't it just another ethernet port? Why not have names like e0 and e1 like you get with a Cisco 2600 series router? I have always learned to define a port by what kind of signals pass through it.

Of course I am aware that you can not treat the LAN and WAN port the same on these little routers. If you could then you could turn a very cheap SOHO router into a very versitle networking device (even without the benefit of Cisco IOS). Why not just use a SOHO router to connect 2 LANs without any care about Internet access like so: LAN1<>WAN Port ---- SOHO Router --- LAN Port<>LAN2. In theory this should work . . . I've tried a few experiments and have not been able to make this work. why?

Thank you for your thoughts.

 

[bcastner]

We are talking about the end point of a DSL/Cable connection, not the specifics before the modem.

The router is Cable/DSL because it can support authentication and lease maintenance on those two connection types. This is handled by the firmware of the router.

For your other question the WAN port could be anything, as long as it is different from the LAN side of the router. To connect your two LANS you would have to tell me a little of the network addressing used. If they are on the same network segment, connect to the uplink port on the LAN side of the router, or use a cross over cable and connect to any LAN segment. If they are on different LAN segments you could use the WAN port with a cross over cable.

 

[MaxPipeline]

While the WAN port is basically 802.3 compliant ethernet, it is the protocols that the router uses over that port that sets it apart. Yes, you can plug the WAN port into a switch and you will get a link LED. Likewise if you plug the ethernet port on the DSL modem. Layer2 and below it is the same.

The difference is the router itself treats the port as a WAN port. Assume for a moment that the WAN port was a T1 or some other telco line. Layer 1 and 2 would of course be different, but layer 3 on up would be the same. I should point out, though, that PPPoE is essentially a layer2 protocol which is probably the one thing that differs from regular PPP which would be used on a telco line. The point is if you think of the WAN port in a logical sense, it is analogous to a T1 or some other type of trunk. It just so happens that instead of T1 it is 802.3 ethernet.

Why do they call it WAN instead of e0, e1? Think about the target market of these routers. Most customers don't know the difference between WAN and LAN and probably never heard of an IP address before. It is in the best interest of SOHO router manufacturers to idiot-proof their routers as much as possible. Cisco routers are really for business and most likely overkill for home use. Most companies have an IT department or at least contract someone for IT. They're use to the industry convention (many of which Cisco developed). Home users are better off with less cryptic labeling.

As for your issue with not being able to connect your two networks together, assuming that you are using the proper cat5 cable and layer1 is working, make sure you have the IP address of the router hardcoded and not using PPPoE. PPPoE cannot receive an IP address via DHCP. Further, again think about the application these SOHO routers are designed for. They are designed for private networks to share a single broadband Internet connection. That means they are designed to run NAT. This means you will need to rethink your network design. Your SOHO router will need to have the WAN IP address be a part of the LAN1 subnet and the LAN IP address should be a part of the LAN2 subnet. Theoretically this should work, though I have not tried it before. Remember also that since LAN2 will be behind the router running NAT, LAN1 devices will not be able to reach any LAN2 workstations, though LAN2 workstations should be able to reach LAN1 hosts. That is unless you set up port forwarding. This is why having a Cisco router per se instead of the SOHO router is more beneficial. You are not bound to use NAT and therefore should not have any problems routing back and forth.

Hope this helps.