Different IP Addresses for the same computer

[dee-u]

Is it possible for one computer to use different IP Addresses when connecting to the internet? I am not a techie so forgive me for asking...

TIA

[penagate]

Depends, if you use a router (most broadband modems) then your publicly visible IP address is that assigned by your ISP to your router, so it is the same for any computers connected to the router at that time. You can only have one at a time if that's what you're asking

[szlamany]

All might have the same "outside" IP address, but if you have several PC's in-house connected to the same router - they will all have different "inside" IP addresses assigned (most likely assigned by the router). The router NAT will translate the "inside" addresses into recognizable "outside" paths so that each PC in-house can surf the web independently...

[penagate]

Yeah.. that's the bit I omitted to mention

But to the outside world for all they know it could be all one PC.

[szlamany]

For what it's worth...

Before we rebuilt our server/network and bought a new router a couple of months ago...

Whenever I VPN connected my workstation to one of my "customers" networks I would experience IP-conflicts. Both my internal IP's and my customers internal IP's all started with 10.0.0.x - which of course caused a problem. Also we both had servers with names like SERVER01 - this also caused problems. I made sure that when I rebuilt the network that my main server would not have a generic name like SERVER01 and that I chose a unique internal range (10.0.117.x) for my inside addresses.

Another point - some of my customers allow me into there networks by "accepting" only the public address that my router expresses. It doesn't matter which machine inside my office is used to "connect" to that customer site - they all appear as the same address (as penagate said).

[CornedBee]

Some web offers (usually those for business usage) give you more than one IP address. My brother's web design company for example, they can use 4 different IPs. A single computer can have more than one of these.
Also, my server has more than one IP internally, where I control IP addresses. There are technical reasons for this setup.

[szlamany]

My server used to have two network cards - a different IP address for each card.

Is that what you mean by a server having more then one IP address?

[CornedBee]

Nope. With two NICs you can be on two networks - my server does that, on the external and on the internal net.
However, it has two distinct IPs on the internal net, for a total of three different IPs.

[szlamany]

Curious - why do you do the two inside addresses and how do you do that?

[CornedBee]

Why? Two reasons.
1) I have both a DNS cache and a DNS server. They must use the same port, so they can't use the same IP. I have the cache running on one, and the server on the other.
2) I need to run both PHP4 and 5. They can't run in the same Apache, so I need two Apaches. Those two can either run on different IPs or different ports. Different ports means I always need to specify the non-standard port, which is bad. (It's the method I use outside, though, where I can't use the more convenient multiple IPs.)

How? Simple in Linux; you just define aliases. In a Gentoo config, this looks like so:

config_eth0=(
        "192.168.1.1/24"
        "192.168.1.2/24"
)

[szlamany]

Oh - interesting...

edit: More grey-matter used for something I might never need to use

[dee-u]

My server used to have two network cards - a different IP address for each card.

Is that what you mean by a server having more then one IP address?

When a computer does have two NIC does it mean it can have two Public IP Address?

[penagate]

If you have two routers, one on each NIC. Or they are both directly connected.

[dee-u]

For instance, if I've got an app that uses the webbrowser control, is it possible that each instance of the app could use different public IP?

[CornedBee]

Not really, because you'd have to have two different internet configurations at the same time. IE is not that flexible. (Nor is any other browser I know.)

[iPrank]

What will happen if I have 2 different internet connections ? Which one IE will use ?