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Hello fellow networkers. I am very new to this world and don't know a great deal about IP addresses and subnetting.

I however stumbled upon quite a big problem (for me at least - haha)

My entire network and infrastructure I have built is all in the 192.168.1.x / 255.255.255.0 range.

Up until now I have had a router/modem that I have always used in modem mode with an IP I could change; said IP would be 192.168.1.254 and would be the gateway which the DHCP (windows server) would use.

Everything worked fine until I had to change the router/modem. This one comes with a fixed IP in router mode: 192.168.100.1 Which makes it unusable for me.

Is there a way to still use it or re-map it somehow in DHCP (windows server)?

I am thinking maybe translating the address, but I don't know how to do it. Ideally I wouldn't change the current configuration, it's a lot of machines and mappings, etc... it would take me a long time!

Thanks a lot for any help!

  • Welcome to Network Engineering! If you can't change the router settings (or get your ISP to do it), you'll have to re-address your devices. – Ron Trunk Nov 21 '18 at 13:21
  • Hi Johnbell and welcome to NE ... if your current DHCP server is giving out .254 as the router address, is there something stopping you changing that to give out the new addresses and gateway? The principal purpose of DHCP is so that you don't have to do any manual reconfigurations. – jonathanjo Nov 21 '18 at 13:37
  • there are several VMs with static IP addresses in a different range (192.168.1.x) and network shares with the same range, DHCP to reconfigure, DNS, reverse proxy... it will be a pain to re-configure everything to match the ISP's IP. And no the ISP won't change it. :( – johnbell Nov 21 '18 at 13:52
  • I understand the problem. If your ISP won't help (personally, I'd start looking for another ISP), you could add another router between your network and the ISP router. but that would also require adding some routes on their router. – Ron Trunk Nov 21 '18 at 14:10
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    Last time I found myself in a similar situation I made myself sign a contract with my future self which said "I promise to use DHCP and DNS in the way intended", so I never had an ugly renumber ever again. DHCP with static leases for all servers, and I have refused to buy devices without DHCP. I'd suggest the pain of the reconfiguration with DHCP/DNS. – jonathanjo Nov 21 '18 at 15:43
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Any serious hardware should be able to have the IP address changed. I'd dump that router.

However, if that isn't an option there are three basic solutions.

  1. renumber - Renumber your network to make it use 192.168.100.0/24 subnet addresses. If most devices are configured using DHCP this is not much more than changing your (few) static devices, updating your DHCP server, DNS server and documentation. If you're mostly using static addresses, now's a good time to reconsider.
  2. route - On the new router, add a static route to 192.168.1.0/24 (or set up a routing protocol) via another router attached to that subnet.
  3. NAT - If everything else fails you can attach a (source) NAT router to the 192.168.100.0/24 network and "hide" 192.168.1.0/24 behind it. Note that this is the ugliest solution - port forwarding/destination NAT would require both routers to separately forward external requests.

So would that act as the gateway ( 192.168.1.1 ) that then would send the request to 192.168.100.1?

Source NAT on an intermediate router replaces any 192.168.1.0/24 address with e.g. 192.168.100.2 on outgoing traffic. The uplink router can't see that there's a whole network behind that address.

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  • In the end I used the NAT and put the hub into modem mode with the Airport Extreme. It worked (inelegantly I must admit), but I'll be getting an Edgerouter Lite for firewall purposes soon and will replace with that. Thank you! – johnbell Nov 23 '18 at 10:27

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