Question about Multiple NICs

If you have more than one NIC installed in a system:

Will one NIC send [passthrough or possibly packet forwarding] info to the other NIC.

NIC1 = Network 1 SubnetMask
NIC2 = Network 2 SubnetMask

A call to an IP from Network 1 to Network 2. Will NIC1 send the call to NIC2?

How can you keep that from happening if it does?

If it doesn't allow the passthru request, how do you make it do so?

Whats the maximum number of NICs allowed in a system [assuming you have enough PCI slots [or ISA =)]]?

Thanks in advance
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More about question multiple nics
  1. Firstly, I think you have gotten your masks wrong, unless you are talking about some very strange access list stuff. However, whether the NICS talk or not is adjustable on almost any OS, but by default may or may not be turned on. IIRC, W2K server has it turned off and many *nixes have it turned on, but then through some sort of a routing process.


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  2. Well, always lots to learn. =)

    How are they wrong... any advice or info can only help me fix my li'l network.

    Also... If you could take a look at my 'Dual Networks Crosstalk Problem' post you might see a little bit more of what I'm trying to work out. =)

    Thanks for the help!
  3. your gonna have to set up your computer as a router if you want it to do that, i dont think it will by default on any system. And yes your masks are definatly wrong, they must be in (binary) 1s and then 0s. a valid mask, say would be in binary 11111111111111111000000000000000. would be 11111111111111111111111100000001. YOu can see why that would not be possible.
  4. Cloaked... In all due honesty I do not see a problem with that. In fact, on this system I'm using right now the subnet mask is Why would be invalid? I've seen instances at several locations whereby we were using masks 255.255.255.x where the x=0,1,2,etc.

    Sorry if I seem dense. Just don't know why these are considered unusable masks if they are working in practice in several locations I've worked at.

    Could it be I'm thinking 255.255.254.x? Or am I still just far off the mark on this issue?

    Thanks for the attempt to educate, Cloaked. =)
  5. I'll try to explain...
    Subnet masks decides what part of your ip address that is used for the network and which part is used for host addresses. Look at the addresses below:

    IP 1:

    IP 2:

    IP 3:

    Simply put the part of the address above the 255s in each address denotes the network. That means that IP 1 and IP 2 can "talk" with eachother, but you'll need to route for them to be able to reach IP 3. Note that IP 3 has a 1 in the third octet. (Octet = nubers between periods, the first octet above is 192 for all addresses for IP and 255 in 1., 2. and 3. octet for masks)
    The numbers above the 255s have to be _exactly_ the same for two hosts to be on the same network.

    The 0s in the mask denotes host addresses and, as long as they have the same network address, any host address can talk to another host address e.g. the last .1 and .2 above.

    If you have a mask of it looks like this in binary: 11111111.11111111.11111111.00000000

    All the corresponding bits in the IP address i.e. the bit above the mask must be the same for 1s in the mask and different for the 0s for two machines to talk (again without routing of course). Now if you have a mask of you'll have a strange network that's difficult to deal with because every *other* host address will be on different networks. Thus IP 1 above will not talk to IP 2. In fact it will only talk to machines on the 192.168.0 network that also has a last octet of an odd number. So IP 1 will talk to 1,3,5,7...253 and IP 2 will talk to 2,4,6,8...254.

    If you have a mask of two its even more difficult to work with:

    2, 3, 6, 7, 10, 11, 14,15... will be able to talk. Try to find the numbers that IP 1 can talk with. Easy right?

    You will run into several problems with this setup since most routers regard these types of masks as invalid and discards the package.

    What you probably have seen are masks that end in 0, 128, 192, 224, 240, 248, 252, 254 or 255. These are all valid (however they are always preceeded by 255 if not in the first octet and always followed by 0s if they are not the last octet). The reason these are valid is because IP addresses and masks are in binary. They are just simplified by dividing the 32 bit number into four octets for ease of use. Here are the numbers translated into binary:

    128 - 10000000
    192 - 11000000
    224 - 11100000
    240 - 11110000
    248 - 11111000
    252 - 11111100
    254 - 11111110
    255 - 11111111

    As you can see they all begin with a row of ones and end with a row of zero.

    Hopefully this explains a little why the masks you have chosen are wrong.

    The easiest thing you can do is to have a mask of and use IP addresses of 192.168.0.x, 192.168.1.x, 192.168.2.x and so on where the x is the host address.

    Lastly you can not use 0 or 255 for the host address as these numbers have special purposes (network address and broadcast address, respectively).

    I don't know if I made myself clear or not, but I hope I explained it a little


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  6. Dev,

    I am studying your post atm...

    Hopefully what little I've managed to learn thus far can be translated here into some useful knowledge to apply to my "Dual Networks Crosstalk Problem" post!

    Lemme see if I've picked up on this correctly...

    The mask defines what bits MUST be [boolean OR] at least in one position of the IP addy.

    For example:


    Ok, in making that example I realized my theory didn't work/apply. Uggg.

    Guess I'll just have to continue re-reading your post.

  7. For simplicity, just use masks of The above post is just trying to explain why x.x.x.1 is a bad mask and you do not need to know or understand it to make a small home network.
    On a side note: you should know is that its always "safe" to use addresses that start with 10.x.x.x or 192.168.x.x or 172.16.x.x since these addresses are assigned to be "private" and will never accidentaly be routed on the Internet (all internet routers reject them).
    Back to the problem at hand. I am a little confused as to what you want to do, so I'll see if I have gotten this right.
    You want to have two networks where only one computer on each can access your DSL connection. You'll actually need three networks to do this, but one will be using DHCP. Using two Nics (alternatively two IP addresses on one NIC) on each of these you can create the networks like this. (Assuming here that you have a DSL router). All have a mask of

    DSL router config: DHCP scope

    Internet PC 1:
    NIC 1: DHCP (Get address automatically) connected to router
    NIC 2: connected to the other machines having addresses of 192.168.1.x (x here is just 1,2,3,4, etc depending on how many ip's you need.

    Internet PC 2:
    NIC 1: DHCP
    NIC 2: 192.168.2.x and increment x for the other pc's on the same network.

    If you want a pc on the 192.168.2 network to reach the 192.168.1 network you'll have to do some routing. This might be achieved with Internet Connection Sharing (ICS), but I am not sure since I never use it.

    The problem with using ICS will be that all computers might be able to reach the Internet, and you will have to block the addresses for these computers in the router. It's not that hard to do, but I understand if it all seems to be a little confusing at first. You'll probably have go through some trial and error, but it feels great once you get it right. I promise :)


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  8. Thanks again Dev... I actually read your last post about 5 or so minutes after you posted. I didn't reply as I am still trying to figure out the subnetting of masks.

    See... I had always thought subnet masks defined the network that would be allowed to communicate to eachother by way of having identical subnet masks. I didn't realize that the masks defined the actual octect number PER OCTECT that would be allowed to comm on the network. Sooo... still trying to get a better understanding of the subnet masking process.

    Yes, I know I don't 'need to know' it all to function... but to function 'better' I do. =)

    THanks so much already, Dev. =)
  9. I have wrote an introduction to networking manual for a class some time ago that I could send you. It is very basic, and its getting old, but the students seemed to grasp the concepts pretty quickly. I can send it to you if you send me your email, just PM me your email address or something...


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  10. If this is on an XP machine, you can easily bridge the 2 connections and that will connect the 2 networks. I had this setup on mine for a while bridging my wireless connection with my Lan adapter.

    wireless router->wireless card(bridged)nic->switch->other computers in my room.

    All the machines then recieved an IP address from the router through DHCP
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