Which LAN gets priority?

Discussion in 'Network Routers' started by Jon Danniken, Dec 1, 2011.

  1. Jon Danniken

    Jon Danniken Guest

    If a PC has two LAN cards, each connected to a DSL router/modem, which one
    does the computer go to for the connection?

    The reason I ask:

    I currently have a DSL modem connected to my computer via a LAN card
    (LAN01). I bought a router to play with (not connected to any WAN), and
    when I connect it to another LAN card (LAN02), the internet goes out on the
    computer for awhile, before eventually reconnecting back to LAN01.

    Even when the internet is working fine on LAN01, any request sent out shows
    momentary activity on LAN02.

    So I am wondering why the router on LAN02 seems to be affecting the internet
    connection on LAN01.

    Any ideas appreciated,

    Jon
     
    Jon Danniken, Dec 1, 2011
    #1
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  2. Jon Danniken

    Char Jackson Guest

    You don't specifically mention which Operating System you're using, so
    I'll assume a recent flavor of Windows.

    From a Command Prompt, check the output of "ipconfig /all" and "route
    print". The first will show you the local interface IP address and
    Gateway IP address of each NIC, among other things, while the second
    command will show you the current routing table, including the IP
    address of the Default Gateway and the metric of each interface.

    Using the output of those two commands, you should see what's
    happening.

    I have 2 NICs in my main PC that are members of 3 different subnets.
    The primary NIC has a single IP address while the secondary NIC has
    two IP addresses. I see no slowdowns and no weird traffic activity
    regardless of which NIC or which subnet I'm accessing.
     
    Char Jackson, Dec 1, 2011
    #2
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  3. Jon Danniken

    Jon Danniken Guest

    Hi Char, sorry, WinXP sp3. The router is an old Linksys BEFSR41 r4.2.
    Ah, thanks, I didn't know about the "route" command. Lots of stuff there.

    What stands out is the bottom-most line from the "route print" command, the
    "Default Gateway" IP. Normally, this is the IP of my main NIC, the NIC
    which connects to my DSL modem.

    When I plug in the router (with no WAN connected) to a different NIC, the
    "Default Gateway" switches to the IP of the NIC that the router is on.
    Since the router isn't connected to the internet, the computer has no
    internet access.

    If I wait a minute and use a browser window to call a website, it will first
    come up as unavailable, but after a few tries it will finally work.
    Checking the "route print" once more shows that the "Default Gateway" has
    once again returned to the IP of the NIC connected to the DSL modem, which
    gives the computer internet access.

    So what is it in Windows that is changing the Default Gateway for the
    computer from the DSL modem to the router?
    Hopefully I can figure this all out so I can function at that level. :)

    Thanks,

    Jon
     
    Jon Danniken, Dec 1, 2011
    #3
  4. Jon Danniken

    Char Jackson Guest

    Cool, thanks. I'm using the same OS, so that makes it easier.
    Take a look in Start - Settings - Network Connections (right-click on
    Network Connections so that it opens as a window or locate it in
    Control Panel). In the Network Connections window, click on the
    Advanced menu and select Advanced Settings. In the dialog that comes
    up, see if the new LAN connection is listed as a higher priority than
    the old (DSL) LAN connection. If so, you can click the arrow to change
    their relative priorities. They'll probably be called Local Area
    Connection 1 (and 2 for the second one). You want the DSL connection
    to be a higher priority than the other connection.

    I think that might give you what you want, but I do a couple more
    things here. My second and third network connections don't have a
    Default Gateway configured at all, so only traffic that's supposed to
    use those interfaces can actually use them. When I want to use either
    of those connections I create a specific route by opening a Command
    Prompt and using the "route add blah blah" statement. In addition, my
    second and third network connections are on different subnets from
    each other, and both are different from my primary connection. That's
    mostly to help me keep everything straight, but it's not a strict
    requirement. The route command will allow you to specify which
    interface to use to reach a specific destination, but in your case the
    biggest thing seems to be the assignment of the Default Gateway, and I
    think the Advanced Settings mentioned above will do that for you.
     
    Char Jackson, Dec 1, 2011
    #4
  5. Jon Danniken

    Jon Danniken Guest

    Thanks Char, I didn't know about that one. Unfortunately, it doesn't have
    an effect on this behavior. When I plug in the router, the "Default
    Gateway" stated from "route print" is given as the IP of the router. It is
    only after a few attempts to call up a website in a browser that the Default
    Gateway changes back to the IP for the DSL modem.

    This works for awhile, until I go to whatever page triggers what I suspect
    to be a timeout, at which point the Default Gateway changes back to the
    router, and I lose connectivity. (my suspicion is that my hosts file is
    causing a timeout with something a webpage tries to load, leading the
    computer to try to find a connection on the router).
    Okay, that sounds interesting. Maybe all I need to do is to disable a
    Default Gateway on the router, but how?

    One piece of information I am observing is on the routing table. The first
    two lines are the only listings for Destination 0.0.0.0, and both the IP of
    the DSL modem and the IP for the router, respecitvely, are listed as the
    interface. The other two NICs I have on this computer (neither of which
    assume a default gateway) are listed for that Destination:

    Network Destination Netmask Gateway Interface
    0.0.0.0 0.0.0.0 192.168.72.1 192.168.72.3
    0.0.0.0 0.0.0.0 192.168.75.1 192.168.75.100


    This listing is present when the Default Gateway given in "route print" is
    either the router or the DSL modem.


    Thanks,

    Jon

    When I want to use either
     
    Jon Danniken, Dec 2, 2011
    #5
  6. Jon Danniken

    Char Jackson Guest

    Ok, I don't remember it working that way here, but it's been a long
    time so thanks for the reminder.

    I think your best bet is to simply remove the default gateway from the
    second adapter. With only one adapter having a default gateway
    assigned, Windows will have no choice but to use that address for all
    non-local traffic.
    You do it on the XP PC, not the router. Each network adapter on the PC
    has its own configuration, usually including a default gateway.

    To remove a default gateway from the second adapter, open Network
    Connections, then open Properties for Local Connection 2. Open TCP/IP
    Properties and select the option to "Use the following IP address:.
    Enter an appropriate IP address and Netmask. Leave the Default Gateway
    field blank. Optionally, leave DNS set to Automatic or "Use the
    Following..." and either enter the IP of a valid DNS server or simply
    leave it blank since you'll only be using this adapter to access a
    locally attached device by its IP address, so DNS isn't required.

    Let me know how it goes.
     
    Char Jackson, Dec 2, 2011
    #6
  7. Jon Danniken

    Jon Danniken Guest

    Perfect! Thanks Char, that fixed this issue.

    This does bring up another question, probably involving subnets (which I
    cannot seem to wrap my head around).

    The IP of the router (set in the router itself as "Local IP Address"), is
    set to 192.168.75.1. Additionally, when I want to view the configuration
    page for the router, I go to 192.168.75.1.

    But in order for the NIC to connect to the router, the NIC has to be set to
    192.168.75.100. If I set the NIC to 192.168.75.1, it won't connect to the
    router.

    What gives?

    Thanks again for your help,

    Jon
     
    Jon Danniken, Dec 2, 2011
    #7
  8. Jon Danniken

    Char Jackson Guest

    Excellent, thanks for confirming.
    Generally, an IP address can only be assigned to one adapter in a
    given network. If two adapters are given the same address, a conflict
    occurs. It's sort of like giving your house the same number as the
    guy's house down the street and the mail carrier doesn't know where to
    deliver the mail. Every address needs to be unique.

    In the case above, the router's LAN IP is 192.168.75.1 (and the
    Netmask is assumed to be 255.255.255.0), so the other valid IP's on
    that network are 192.168.75.2 thru 192.168.75.254. Generally, every
    network also has two reserved addresses, which in this case are
    192.168.75.0 (which refers to the network itself) and 192.168.75.255
    (which is the broadcast address for that network).

    So if you have a device with an address of 192.168.75.1, it makes
    sense that you can't assign that same address to a second device on
    that network and expect it to work. In your example above,
    192.168.75.100 is a valid IP address for that network and it will
    enable you to talk to the router, but any address between 192.168.75.2
    thru 192.168.75.254 will work just as well because they all lie within
    the Netmask.
    No problem, I'm glad you made serious progress.
     
    Char Jackson, Dec 2, 2011
    #8
  9. Jon Danniken

    Char Jackson Guest

    Keep in mind that you don't necessarily need multiple NICs to join
    multiple networks. You can assign multiple IP addresses to a single
    NIC and the OS will automatically use the correct source IP for the
    network you're attempting to access. That comes in handy when you want
    to be able to access more networks (I mean access directly, not via a
    router) than the number of NICs that are available.
     
    Char Jackson, Dec 3, 2011
    #9
  10. Jon Danniken

    Jon Danniken Guest

    Thanks Char, that makes sense now. Just for fun I changed the static IP of
    the NIC from 192.168.75.100 to 192.168.7550, and it still worked, which was
    a good confirmation of the concept.
    Indeed I did, and I also learned a lot more in the process, which will come
    in handy when I actually hook the router up to the modem.

    Jon
     
    Jon Danniken, Dec 3, 2011
    #10
  11. Jon Danniken

    Jon Danniken Guest

    Interesting, thanks for that. I dug around in TCP/IP properties, and found
    a place to enter more IP addresses in the "Advanced" tab.

    Jon
     
    Jon Danniken, Dec 3, 2011
    #11
  12. Jon Danniken

    Char Jackson Guest

    That's obviously 192.168.75.50. I knew what you meant.
    It's fun, right? :) There are tons of great tutorials online. I like
    to point people to www.practicallynetworked.com as a good place to
    start.
     
    Char Jackson, Dec 3, 2011
    #12
  13. Jon Danniken

    Jon Danniken Guest

    Aye, it is indeed a fun puzzle to figure out. Can be frustrating at times
    when you don't have another computer in the house, and botching up your only
    network connection leaves you high and dry, though, but that's part of the
    excitement.

    I have found some tutorials online, but I still like usenet for some reason.
    I'll probably make another post here pretty soon to confirm what I think is
    the right way too hook everything up.

    Jon
     
    Jon Danniken, Dec 5, 2011
    #13
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