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So I found out that the cable company use the DHCP to assign the public ip address dynamically. So a subscriber's public facing ip address can change during the length of their subscription. Now what if you remove DHCP on a particular computer, which you plan to use as a web server, so that the machine has a static, unchanging IP address. If the public ip address was to change, would this confuse the Network Address Translation (NAT) and cause some sort of disruption? Please answer in layman terms, as I'm still grasping concepts here. thanks.

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closed as off topic by John Gardeniers, Ward, Bart De Vos, Michael Hampton, Scott Pack Sep 23 '12 at 0:57

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2 Answers

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Um, what?

It's kind of unclear what you're asking here, but to put it as simply as possible, if you put a server (or network or whatever) on public facing IP that changes, when it changes, anything accessing your server (or network or whatever) will lose the connection and be unable to reconnect, until such time as the updated [correct] address is propagated.

Typically a name server of some sort is used to facilitate this (DNS or WINS, for example), which would translate your server name to an IP address. So, supposing it's a webserver (because that's the simplest example for this), when someone types www.yourwebsite.com into their browser bar, their browser checks against their ISP's DNS server to get the ip address of your site, and connects to that. If your IP address has changed, they'll still be able to get there, so long as you've updated your DNS records, and given time for the DNS record to propagate.

(How long that takes depends on the "quality" of your DNS provider - anywhere from a few days to a few minutes, depending on what level of service you're paying for).

If they try to browse to your website before their DNS gets the updated address, they will end up connecting to the old address and whatever (if anything) is actually located there.

Obviously, if you have people connecting by IP, and the IP address changes, they will need to manually update their records...

Regarding NAT, I'm thinking you're (at least) a bit confused on the concept, so maybe specifics would help, but generally speaking you deal with NAT on network gear like a firewall or router, and not on a web server. For example, if I had a public address space of 5.5.5.0/24, I might decide 5.5.5.37 is going to be the public address of my webserver. I would then go to my firewall (because I'm not going to run a public-facing anything without firewalling it) and set a NAT rule to say that any traffic going to 5.5.5.23 should be directed to whatever private network address my webserver resides at, like 10.12.68.210. In that way, people would be able to access my webserver from the public internet, even though it's actually on my network, in a private address space.

That's the function of NAT. Not really about DHCP or dynamic addresses, per se... though if in my example, my ISP goes crazy and reassigns my address space to 5.5.6.0/24, I'd have to update my NAT rules (and my routers) to correspond to the new address space, which I'd do as soon as I was done firebombing my ISP.

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simple answer no.

The nat on home based router works with a default rule of all incoming packets with a source address of xxx.xxx.xxx.xxx and a destination address of yyy.yyy.yyy.yyy(your public ip) will be translated to a destination ip that initiated the request on the local LAN.

Therefore will this stop your web server from responding to new requests that come in from the new public IP...no. But the external DNS servers that say yourdomain.com is at public ip yyy.yyy.yyy.yyy will need to be updated to the new IP before new requests come in since the requesting devices wont know what IP to go to unless it is updated in DNS.

What this will may disrupt is sessions where the remote device is strict with session.

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