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Modem mode and MAC address changes - impact on IP address

Technical question but here goes (someone may know/have done this!) 

My current setup sees the hub put into modem only mode connected to pfSense as my router. I'm changing over pfSense boxes (moving to something slightly more powerful for DPI). As there's going to be a new NIC (with a new MAC address), what will happen to the IP address that I currently have going?

Completely understand that the IP address isn't static (but 'sticky' in a sense). Is the IP address paired with the pfSense WAN port MAC address, or the hub MAC if in modem-only mode? I'd rather not copy over/spoof the old MAC on the new box but can do if that's the only way to keep my current IP as I sometimes VPN into my lab from work! 

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Re: Modem mode and MAC address changes - impact on IP address

To keep your current WAN IP you need to copy the MAC over.

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Message 3 of 9
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Re: Modem mode and MAC address changes - impact on IP address

The WAN IP address will change. In modem mode, the VM DHCP servers effectively 'see' the MAC address of the downstream device attached to it, in your case the pfSense router - change the NIC and you change the MAC address and hence get a new IP address. You'll need to clone the MAC address if you absolutely have to keep the same IP, personally I'd just allow the address to change and reconfigure your VPN tunnels - it's not like it will change that frequently.

John

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Re: Modem mode and MAC address changes - impact on IP address

Ah, brill. Might just swap the entire NIC over and keep the new one as a redundant/cold spare. Thanks! 

EDIT: the desire to keep the same one insofar as possible as I've asked IT at work to add in a VPN tunnel on my work devices, which was a bit of an uphill battle so I could loop back to home if needed. As I don't have admin rights on devices (supervised), I'd be stuck. The nice bloke who knew what to actually do is on shared parental leave for the next few months so...

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Re: Modem mode and MAC address changes - impact on IP address

What you could do is setup a DDNS and have the server VPN match your IP by FQDN lookup that way if your IP changes the VPN server firewall updates to the new IP.

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Re: Modem mode and MAC address changes - impact on IP address

The IP address may well not change. I run in modem mode and changed from one router to another, different MAC addresses, and my WAN IP address did not change.


Tudor
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Re: Modem mode and MAC address changes - impact on IP address

Why would changing devices behind the hub in modem mode make any difference? It's the negotiation between the modem and the CMTS which results in the WAN IP address being served.

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Re: Modem mode and MAC address changes - impact on IP address

You sure your IP didn't change with a different MAC @Tudor ? Maybe VM locked the number of allocated IP's in your area so tight that it gives you the same IP after a ARP scan to your old MAC gets no reply frees up that IP and you get the same IP again on a new MAC? And thats just one theory!

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Message 9 of 9
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Re: Modem mode and MAC address changes - impact on IP address

This was my thought too. I remember when I first got my VM install done and put the hub into modem only mode, I was assigned an IP that ended in .62 - after following the procedure of switching off my Hub, letting my pfSense box boot, then switching the hub on, I got the IP address I've got now from the same CIDR range. I think its to do with the request that comes from the client (ie the pfSense box) passing through the hub to VM's kit - I'd expect it to send a MAC identifier with the request.

@legacy1 is probably on the money in your situation I'd say on being able to move MAC and keep the IP - as long as no one else does the same while you're swapping... 

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