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Robert LoganFlag for United States of America

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How to get static IP from an ISP using a switch. Possible?

The ISP I use, xFinity, charges $100/month (arm and a leg) to provide me with a static IP for my home Windows network, specifically a domain controller.  I'm wondering if there's a workaround.  Could I connect a switch to their router that would function as a DHCP server and serve out a static IP address to my Windows domain controller?  Bottom line?  I'm trying to save $$$.
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David Johnson, CD
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If I'm interpreting this question correctly, you want to ensure a static IP from your ISP and NOT on your internal network without paying.

If that's the case no, putting a switch on your internal network and having that get the same IP won't work. They are dished out by the ISP DHCP server and not your internal network and as such, chargeable. If it's for internal only, then  David is right.

Why do you need a static IP? Did you look into using a dynamic DNS provider?
Are you asking for a static public IP or static private IP for you DC?

DCs should not be accessible from the Internet, and should use static private IP (not DHCP). In fact, your DC should be the DHCP server for your network.

I don't see why you would need a static public IP, and behind a router your ISP doesn't care what you're doing.

Please explain why you think you need a static IP and what it will accomplish.
The short summary, Is a switch does not have DHCP functionality built-in.
The $100 you are paying will remain unless you modify the plan you have no matter what you do on the LAN side.

It might be easier as the other commenters suggestion to get a clearer understanding on what your environment is and how it is being used.

Unless you have multiple locations thus needing them to have a way to connect to your main location, a static IP from xfinity might not be needed, but the fee you are paying is more likely related to the speed for which you signed up versus the type. I think they only charge $5-10 for the static IP per month.

Do you use VOIP type phones not from Xfinity that rely on the internet connection speed.

Usually, you have access into the router that they provided. and if you need to specify that you want one or several systems to have a static IP on the LAN side, you could configure the DHCP server on the router to allocate IPs from a specific range while configuring your DC to use a Static IP on the LAN side that is outside the allocated range.

i.e. x.x.x.50-100 is the allocated range
you can assign an IP to the DC on the x.x.x.2-49 if you use 1 as the default gateway which is the common.

I believe you should have full admin access rights into the router to manage the LAN network. including if you use DHCP Reservation.
Look at the connected devices, identify the DC and change it to static within the router interface, it will assign the same IP as it currently has until the router is replaced when needed.
Ask the ISP, what IPv6 costs..
PArdon if I'm repeating a bit here, but there are a couple of important points that I wanted to be sure you understand.

If you need a static IP address on the internet, those are handed out by the organizations that "own" them. So you can't just pick one of your own and expect it to work, nor can you take a dynamic IP for the ISP and assume it will stay the same.

As mentioned, there are other companies that offer you a dynamic DNS service that might fit what you need.

Most important, your network should not use a public IP address. That's why you need a router (as opposed to a switch). The routers uses you public IP on the outside, and route traffic to whatever internal range you are using (i.e. 192.168.1.x).
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My goal is to set up a test/home Active Directory network with a single domain controller and two Windows 10 workstations.   For clarity, here are more details:

 - I have a basic Internet service plan with xFinity.
 - The xFinity router assigns DHCP addresses to my three computers via LAN cables.  (Actually, a single LAN cable from the xFinity router goes to a mini-hub and three LAN cables from the mini-hub go to my three computers.)
 - So far, I haven't promoted my Windows Server 2019 computer to a domain controller.
 - The computers on my home network need to access the outside world in order to download updates, etc.
 - In order for my W10 computers to access my Windows Server 2019 computer on my home network, I need a static private IP address for this server so my two W10 computers can always reach it.

Challenge: The xFinity router assigns DHCP addresses to my three computer which change depending on the DHCP lease.

- Set up a test/home Active Directory network for three computers, one WinSrv2019 (domain computer) and two W10 computers.

Should I just assign private IP addresses to all three computers for the sake of learning Active Directory and then change them back to DHCP when I need to connect to the Internet?
So far, I haven't signed up for the Business plan from xFinity which would allow me to get a static IP address.  So, I'm not out $100/month yet!
You're not quite understanding what is going on.
Your isp assigns dynamically a public ip address (depending on your plan you can get for business 1,5,10 public ip addresses)  (these are your
 WAN (wide area network addresses) which goes to the cable box which contains a rudimentary router. Most have 4 LAN ports and another  dhcp server.  This dhcp server gives out addresses in the private range

Range from to — a network with a or an /8 (8-bit) mask.  Range from to — a network with a (or a 12-bit) mask
A to range, which is a network masked by or /16. A special range to with a or /10 network mask; this subnet is recommended according to rfc6598 for use as an address pool for CGN (Carrier-Grade NAT).  

On any machine connected to the LOCAL area network do an ipconfig  and you will see the address that is returned by the dhcp server in the cable box it will be in the private ranges noted above.  DHCP addresses are allocated from the lowest # to the highest # and addresses are reused if the lease has expired otherwise the next higher address is assigned. You could use set statically the address for your  server. without changing anything on the cable box/router.  The disadvantage is that every computer that wants to use the server for AD will require a manual change of the DNS server entries in the network adapter.  The only DNS server address you put in the manual boxes should be DNS servers on your local network.
Presumably you would use your windows 2019 as a virtual host that will have two VM or a host and a VM?

Look on the router, you can control what it does with DHCP, if you want, you can turn off the DHCP on the router and configure the windows based DHCP server to control the LAN side as you need and as David and others pointed out.

Consider it as the follows, the LAN side is your car. The Xfinity service is the road, bridges, highways etc. .
You can do whatever you need within your car. It does not depend on the roads you will take.

The only time one needs an external Static IP is when they are providing services to the Internet systems.

The issue is configuration of the system whether they do or do not have a connection to the internet.
You can as you asked setup your test environment without connecting it to the xfinity router with a switch while one of your windows server 2019 is functioning as the DC, DHCP and a DNS server
Hi David -- If I change the DHCP IP on my server from  to the static IP,, how will it be able to reach the Internet?  Should I add a second network card to my server for the static IP and leave the first network card using DHCP from the xFinity router?
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Arnold -- Yes!  I contacted xFinity tech support and was able to get the user name and password to their router.  I logged in and was able to to set static IPs for the three computers on my home network --

W10-1 (workstation) =
W10-2 (workstation) =
WinSrv16 (AD domain controller) =

xFinity told me these IPs would revert to DHCP addresses if their router was rebooted but I can easily change them back to my static IPs.

Thank you everyone for your explanations.  Very helpful and I've a few things.

The IPS will only revert on a reset to factory of the gateway. If it is power loss or or a reboot the configuration you have set will remain/survive.
You could create a backup of the configuration such that should it reset, you can import it back.
You should really just disable DHCP on the xFinity gear and use DHCP on your DC. If you can't do that, put your lab behind another router where DHCP is disabled on the LAN side.
I would like to express my thanks to everyone who carved out time to address my questions.
f I change the DHCP IP on my server from  to the static IP,,
by using the gateway ip address to access outside of the local area network guessing