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I have a server with multiple IP addresses created though the OS. There is only one network card on the system and one mac associated with the first IP. What is technically going on here? how dose the network see these addresses?

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up vote 7 down vote accepted

Inside the local (ether)net, your NIC will just answer the ARP request for every IP address it has assigned, so its MAC address is associated with multiple IPs. Since the MAC address is all that counts on the physical layer of an ethernet, this is sufficient. From outside the local network, the gateway router will accept the packets on behalf of all IPs in his particular subnet and then forward them to the appropriate machine, again using the physical layer of the Ethernet with arp requests and all.

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You can have as many IP addresses as you want per network card. IP is at a higher network layer than your MAC address.

Your MAC address is not "associated" with any IP addresses. Your MAC address is associated with your network card.

It probably helps if you open a command prompt and type ipconfig /all which will list all the network information. The listing will be per network card.

When a computer on the network wants to find your server, it sends a broadcast request saying "hi everyone, who has this IP address?" and your server will reply "I do, you can find me at this MAC address." Because it works this way, there's nothing stopping your server from answering to multiple IP addresses.

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An IP address is just a label - when a NIC sees a packet that is addressed to one of its IPs (labels), or to a broadcast address (another label), it grabs it off the wire and sends it over to the IP stack. It can listen for more than one.

If you want a lot more detail, start reading up on the OSI model, with particular respect to layers 1-3.

Edit - I presented this a little incorrectly, it turns out. I was really describing what happens with ARP and MAC addresses. So, I guess it's been a while since I spent much time below layer 3 myself.

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An interesting side note on this, at least to me.

We were replacing a firewall with new hardware, it had about 20 additional addresses on the interface on the internet side.

After the swap, we could only reach the main IP for the firewall, not the additional aliases.

We went over everything we could think of, checking all the configs, making sure it was all there.

In the end, turned out that aliases have some sort of lower significance and if you introduce a new NIC with those same aliases, other devices on the network won't necessarily switch over.

This happened to be like 3am at the time and the router our firewall was connected to wasn't ours so we couldn't get on it to reset the ARP cache nor physical access to power cycle it.

I came up with the idea of one by one setting each IP as the main IP on the NIC, bringing it up, let it update the router's ARP cache, then move on to the next. After doing so, they worked as aliased IPs.

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