Allowed IP addresses for two network cards

**swerdna** · 27-Aug-2008, 04:36

Suppose I have two network cards in a computer.
I understand they can't have identical IP addresses.
I understand they can be quite different, like this 10.0.0.3 and 192.168.1.3
I don't understand where the limit lies in between.

Can they be like this example: 192.168.1.7 and 192.168.1.8
Can they be like this example: 192.168.1.7 and 192.168.2.7

Where does the line get drawn?

**ken_yap** · 27-Aug-2008, 05:11

You can have two NICs in the same subnet*, but if you do, they have to be connected to the same (V)LAN. If they are on different subnets, then they should be on different (V)LANs.

* Yes you really can. I have a few servers that are set up like this. It's not very different from assigning two addresses in the same subnet to a single NIC except that you get more throughput (though a Gb card upgrade would be far more effective). Why? Sometimes you need a second or higher address to run HTTPS servers which are tied to particular IPs, or to run some services on one address and not on the other. But in the case of my servers it wasn't planned that way, but since the server came with two interfaces, why not put the second one to use?

**BoloMarkIII** · 27-Aug-2008, 06:44

My mobo an Asus M2N Sli Deluxe has 2 NIC cards are active on my dual boot of Win XP x64 & Suse 11 x64.

The mobo documentation states that the duel cards are hardwired for redundancy if 1 has problems traffic will switch to the other. There are no options in the BIOS to control this function only enable or disable the cards.

In Windows running Suse in VirtualBox I can assign 2nd card to the VM.
Likewise in Suse running Windows in VB, but Linux apps unlike windows apps are more likely to have an option to use 1 or the other card for example ktorrent.

**swerdna** · 27-Aug-2008, 13:51

Originally Posted by ken_yap

You can have two NICs in the same subnet*, but if you do, they have to be connected to the same (V)LAN. If they are on different subnets, then they should be on different (V)LANs.

* Yes you really can. I have a few servers that are set up like this. It's not very different from assigning two addresses in the same subnet to a single NIC except that you get more throughput (though a Gb card upgrade would be far more effective). Why? Sometimes you need a second or higher address to run HTTPS servers which are tied to particular IPs, or to run some services on one address and not on the other. But in the case of my servers it wasn't planned that way, but since the server came with two interfaces, why not put the second one to use?

Thankyou. I tested in Suse 11.0: I have two cards (wireless and wired) and a standard wireless router (with 4 wired ports). I used DHCP from the router with both cards turned on and that works fine, giving consecutive IP addresses. And I tested with fixed IPs on both cards, set to the same IP, which works fine too. I'm surprised because I thought in version 10.x of Suse that it wouldn't work, but probably I'm wrong.

OK I need to completely rewire my thinking so let me ask explicitly:
Q1: is there any reason not to have two cards in the one computer on the same IP or consecutive IPs for a SOHO LAN situation?
Q2: is there any reason not to have two cards in the one computer on adjacent IPs (e.g. 10.0.0.4 and 10.0.3.4) for a SOHO LAN situation?

Thanks

**ken_yap** · 27-Aug-2008, 15:06

Originally Posted by swerdna

OK I need to completely rewire my thinking so let me ask explicitly:
Q1: is there any reason not to have two cards in the one computer on the same IP or consecutive IPs for a SOHO LAN situation?
Q2: is there any reason not to have two cards in the one computer on adjacent IPs (e.g. 10.0.0.4 and 10.0.3.4) for a SOHO LAN situation?

1. You might not want to waste a second NIC since you can achieve the same effect by assigning an alias IP to the first NIC.

2. Those are not adjacent IPs. They might or might not be in the same subnet, which is what matters. If they are in the same subnet, same answer as 1. If they are on different subnets, then you have a router and the NICs should not be connected to the same LAN.

**heffm** · 27-Aug-2008, 15:12

basically the only reason you would need two network cards is for redundancy, its not necessarily going to make anything faster. However the best way to set this up is to have consecutive IPs, I.E. 192.168.1.128 -> 192.168.1.129 having 10.10.10.12 and 192.168.1.28 is possible but they need to come from two separate networks. which may cause some confusion on your computer.