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This is my first question on Network Engineering SE, so I apologize if it is not up to par.

I am trying to help a local business with their network setup, and I have hit a road block. Basically, they have a piece of software that requires a "server" computer to have the IP address 192.168.127.1. Subsequent "client" computers are numbered 192.168.127.2, etc. and all have a subnet mask of 255.255.128.0.

The issue I have come across is that they recently switched Internet providers, and their new gateway/modem is set up as 192.168.2.1. Since each machine on the network has a static IP address on a different subnet than the gateway, they are unable to communicate with the Internet, although they can still communicate with the LAN.

My initial thought was to just change the local-facing interface of the gateway to 192.168.127.1, but that idea was shot down by the requirement of their software, so 192.168.127.1 is already taken.

I then thought perhaps I could change the subnet mask of the gateway to 255.255.128.0, which (I think) would put the gateway and the computers on the same subnet. However, the gateway only allows 255.255.255.0, 255.255.255.128, and 255.255.255.224 (or something very close to that last one).

So currently, we are basically stuck in an either/or, where they can either be on the LAN using a static IP address and communicate with their software, or use DHCP and be able to browse the Internet, but without the ability to access the LAN.

The strange thing is, using the static IP addresses some sites still works, such as Google. It appears that IPv6 is still able to work, but not all sites support IPv6 yet, so it is a very limited subset of the Internet.

I have tried manually entering an IPv4 DNS server, but that also has not worked. I am no networking expert, so this may be a very elementary question, but I am hoping that someone here may be able to shed some light on this situation.

closed as off-topic by Ron Maupin Aug 14 '17 at 2:44

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "NE is a site for to ask and provide answers about professionally managed networks in a business environment. Your question falls outside the areas our community decided are on topic. Please visit the help center for more details. If you disagree with this closure, please ask on Network Engineering Meta." – Ron Maupin
If this question can be reworded to fit the rules in the help center, please edit the question.

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The router needs to be the gateway for the hosts, and the gateway address needs to be in the same network as the hosts. The router should be addressed with the same address that the previous router had, which will be in the same network as the existing hosts.

If the router will not accommodate a valid network mask, I have to assume it is a consumer-grade router, and questions about those are off-topic here. It should be replaced with a router which can be correctly configured. As a temporary workaround. You can set a shorter mask than was previously configured, e.g. /24. Just realize that any hosts added with an address in which the last octet is greater than 126 will not be able to communicate with existing devices, or vice versa. Also, anything which depends on using the 192.168.127.127 broadcast address will not work. Also, if the network mask is shortened, the masks on the hosts could be changed to match.

This is all for IPv4. IPv6 doesn't work the same way, and the IPv6 network will be a /64, which allows for 18,446,744,073,709,551,616 hosts.

  • It is a Comcast modem, I believe Cisco brand. The subnet mask is just a dropdown menu with the three options I mentioned, so I can't specify anything else. Part of the problem is also that there used to be a firewall in between the gateway and the local network, so the firewall could have an IP address on the network with the gateway and route traffic accordingly, while leaving the static LAN addresses alone. – dub stylee May 11 '16 at 23:09
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    Unfortunately, questions about consumer-grade equipment (the Cisco cable modem) are explicitly off-topic here. Whatever had an address in the network, whether it was the old router or firewall, that address can be used for the new router. – Ron Maupin May 11 '16 at 23:14
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First, let me rant against clueless software developers who hard code IP addresses or require fixed subnets, as in this case. There is a special place in hell for them.

(End of Rant)

You will probably have to go out and buy a new router (gateway/modem) that doesn't preset subnet masks. Any commercial grade router will do, and you should ask your ISP for a recommendation.

Set the new gateway address to 192.168.127.126, assuming that address is unused. The mask will be the same as the others, 255.255.128.0.

Change the default gateway on every client (and server) to the new gateway address. If you find that time consuming, you'll understand why smart people use DHCP.

Set the DNS server address on the clients to the ISPs recommendation, or use a public server such as Google (8.8.8.8).

After you get everything working, go back to the software developer and kick them in the shins for me.

  • I agree completely about hard-coding IP address requirements. It sounds like they will just have to be stuck in the either/or scenario until they can replace the firewall or something. Thanks for the tips. – dub stylee May 11 '16 at 23:19
  • Unfortunately, the developers are based out of Italy, so I won't be able to kick them in the shins. Although a trip to Italy would be nice :D – dub stylee May 11 '16 at 23:22
  • @dubstylee, by the way, the cable modem is not a firewall. It can NAT, and that is something entirely different. They should not proceed without a real firewall. – Ron Maupin May 11 '16 at 23:33
  • I know the modem isn't a firewall. The firewall that they had is out of commission right now and needs to be reset/reconfigured. Thanks for pointing it out though. – dub stylee May 12 '16 at 0:10

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