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Are network cards and IP addresses a one-one map?

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Easy example: IPv6 will often have a link-local and a global scope address. –  Steve-o Sep 16 '11 at 15:43
    
Also, all network cards obviously listen on both the external-ip and also 127.1 –  djangofan Sep 16 '11 at 22:26
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@djangofan, you'd be hardpressed to find an OS where the 127/8 range makes it to any level of hardware or driver. They all recognize that address internally, some assign it to a virtual loopback device. –  Chris S Sep 16 '11 at 23:57
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7 Answers

up vote 2 down vote accepted

On linux, the command is ip addr add IFADDR dev STRING, where IFADDR and STRING are replaced by the IP address and device name. You can see some examples in the Guide to IP Layer NEtwork Administration with Linux. You'll also want to specify your routes that use that IP address using ip route. Again, the guide comes through.

Even though the syntax of the ip commands makes it seem that you are associating the IP address with a specific network card, linux considers the IP address as belonging to the overall system. Thus in some circumstances you may want to twiddle with ARP settings.

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Yes, you can have multiple; they are not one to one.

A single network card can do this in a couple of ways; it can answer for multiple IP addresses with a single MAC address (assigning multiple addresses to a NIC in most operating systems will do this), or with multiple MAC addresses (virtualization platforms behave in this way).

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How is what you mentioned configured? –  kernel Sep 16 '11 at 15:24
    
@kernel What OS? –  Shane Madden Sep 16 '11 at 15:26
    
Linux,but other OS should be similar IMO. –  kernel Sep 16 '11 at 15:29
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@kernel - the mechanics of network configuration and "other OS" are completely different. –  EEAA Sep 16 '11 at 15:38
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@kernel. Yes. Google: ip alias {insert OS here} should tell you all you need to know. . –  Doon Sep 16 '11 at 15:51
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Not only is this possible, it is very common. A network card may have multiple IP addresses. This is a concept called multihoming. There are other variants of multihoming as well.

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A single physical network card definitely can have multiple IP addresses. This is almost essential when implementing server virtualization: with multiple virtual servers running on the same physical hardware, each needs its own IP address.

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Yes you can have more than one IP address when using a single Network Card.

Setting this up is different in each Operating System, but may involve creating a new Network Interface. This can look like a unique connection but will be using the same Network Card behind the scenes. OS X and Linux use this method.

To add a new IP address in Windows open up the network connection you wish to add another IP address to. Go to Properties –> Internet Protocol (TCP/IP) –> Advanced –> In IP settings, click add and add the new IP address and netmask.

This can be used to expose a server in two network ranges. For example one network range may be used for secure traffic which the other network range cannot access. Firewall rules could be different for each Network Interface.

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Linux is the only OS I know of that makes you setup a subinterface. Windows and the BSDs do not. –  Chris S Sep 16 '11 at 20:57
    
Thanks for the information, I've updated the answer to be more accurate. –  Thomas Coats Sep 16 '11 at 21:09
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IP layer (the ip address) is completely independent from the physical layer (your network adapter). If your operating system has properly implemented the OSI stack or some close model of it you should be able to easily define several incoming IP addresses on a single network interface. Handling incoming traffic over multiple IP's is pretty easy within linux. However, it's a little more tricky making outgoing traffic have different source IPs when sharing a single interface. Applications typically default to use the primary IP for outgoing source if you don't do anything special to force another IP to show up, but it's definitely possible to do this as well.

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Agreed. Network cards and IP addresses have no close tie. You could have IP addresses with no network cards (virtual networks or SLIP) or network cards with no IP addresses (Appletalk), or 1 IP address over many network cards (Bonded connections) or many IP addresses talking over one card (as other answers). –  TessellatingHeckler Sep 16 '11 at 22:12
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A very common example of one NIC with multiple IP addresses is where the NIC provides a management interface in addition to what the OS sees. e.g. HP servers with ILO.

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