Why would a machine have two NICs?

Just was wondering, but why would a machine, such as one running OpenSuSE have two Network Interface cards?

There are a lot of reasons to have multiple NICs:

Increased throughput via bonding
Access to different physical (or virtual) networks for different types of
Internal stuff
Storage stuff (iSCSI or FCoE)
Public stuff
Backup stuff
To act as a router or otherwise connect different networks
To access different types of media (wires vs. wireless signals)

Good luck.

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Used as a firewall, squid proxy, router etc. One for local network, one for a management network. It could be a LoM (Lights out Management) interface.

My desktop has three NIC’s and a wireless card the NIC’s are bridged for my KVM virtual machines.

It could also be a faster network card eg 1000Mbit instead of a 100Mbit…

Previous posts described many good reasons,
You might also consider that a typical laptop has two built-in NICs for obvious reasons (wired and wireless). Although a typical User would only use one at a time, it’s possible to make use of both when configured properly (often wrongly).
Also consider when someone connects directly to another computing device, like a phone. The connection between the PC and the phone is a connection which might be serial or BT, and if you look closely sometimes you’ll find ethernet running on top of the stated protocol.

Probably the main thing to remember is when you use more than one NIC improperly…

  • When both NICs are connected to the same network, but not configured for bonding. Some might think it improves throughput, without bonding it doesn’t and actually will cause degradation.


The openSUSE Firewall has the typical (per NIC) Use-Cases for this: “External Zone”; “Internal Zone”; “Demilitarized Zone (DMZ)”.

Where, a DMZ is used for machines which need to provide services [Web Servers are a typical example] to the “External Zone” (i.e “the Internet” or, a “WAN”) but, cannot themselves access the “Internal Zone” (usually, the local LAN).

The decision which needs to be made is, buy a couple of NAT (Network Address Translation) Routers or, use a couple of “to be retired” machines with (at least) two NICs? The network management effort required to set-up and maintain the access policies is about the same for both cases but, the machines with two NICs may offer more flexibility.

I remember two systems running in a hot switch over configuration. There was a crossover ethernet between the two for the heartbeat.

Just to point to yet another use case.

I remember also an application where the application server used a database server. Connecetion again betwenn dedicated ethernet connections on both machines with a crossover cable.

I have one of my main machines set up with a Gigabit NIC on ethernet and 3 wireless adapters.

I have a friend who lives here whom I set up with openSUSE, and we share the high speed internet.

I have several neighbours who I have set up running openSUSE, two of whom are non-tech and not interested in learning, so have me administering their machines, and who live within wifi signal range, both on their own internet connections.

On the internet I share with my friend, I have set the modem/router into bridge mode and turned off the wireless signal. I have lent my friend one of my routers and I am using a separate router. This keeps his network and his Facebook garbage, et al, isolated from my own network for obvious reasons.

On administration/tech day once a week, I:

  • Run zypper up on this machine;
  • Connect to my other machines through ethernet using SSH and simultaneously run zypper up on all of my machines;
  • Simultaneously connect wireless to the router I lent my friend and run zypper up on his machine;
  • Simultaneously connect my other two wireless routers to the two neighbours’ routers and run
nmap -sn xxx.xxx.xxx.xxx/xx

to see if their machines are turned on and connected. If so, I use SSH to run zypper up on their machines.

Slick, convenient, and quick.

Just one of many ways to put multiple NICs to use, as you have read by now.:wink: