We have a customer that has a security system that communicates via IP to the monitoring station.

Yesterday I went to site as the customer changed ISPs and now the system isn't communicating to the monitoring stating (Showing a com trouble on the system). I figured it was due to the new ISP using a different subnet than the old company, and confirmed this.

New ISP's DHCP range was set from 2-254 and I adjusted to be 2-100 as we like to keep static IP's for all our devices to make connections and port forwarding easier.

I set the new IP address on the security system to be 110 and am not getting an internet connection to the security system. System is still unable to communicate with the monitoring station.

Contacted tech support for the security system and they said that there is nothing wrong with the system and the new ISP's address shouldn't cause this trouble.

I contacted the ISP tech support and they informed me that in order for a device to gain access to the internet it must use the DHCP. Any device outside the DHCP range will not be able to obtain an internet connection. I have setup many systems using ISP routers as well as installing our own routers and have never had an issue where a device with an IP address outside the DHCP range of the routers was not able to get an internet connection until yesterday.

Can anyone provide any reason why this is the case? ISP's router is a Comtrend AI-5720u.

Try this-

http://www.canyouseeme.org/

Try setting port 80 to a different number.

This has nothing to do with forwarding ports. This allows remote access to the security system. For some reason the ISP's router is blocking outgoing connections on all devices that are setup outside the DHCP range of the router.

Put another router between their POS turdbox and the rest of "your" network?

does the ISP router allow you to reserve addresses?

Yes I believe it does, something we have never used before as I would prefer to use static IPs rather than DHCP reservation.

For the three devices that are connected to the router I can't justify installing a router additional to the one they already have.

in the end it accomplishes the same thing.  that is assuming that your settings don't get wiped out by an update pushed to the router by the cable company.

you could reserve the address, and set a static address.  I would do this towards the end of the dhcp pool assuming the router hands out the lower numbers first.

How can I set a static within the DHCP range? From what I can tell the router is handing out IP's starting at the lowest number.

What CAN you justify? :P

Bustin your balls. You obviously need a solution. Any solution will cost somebody some time at the least. For only 3 devices, any off the shelf router from Best Buy should be sufficient IMHO. The $99 or so may be well worth it.

DHCP reservations are a good idea as well. But as mentioned, the ISP could easily wipe those out at any time with a reset, update, replacement modem/router, etc. requiring another visit from you.

Do the 2 other devices have static IP addresses as well? Or just the security system? Does it have to have a static? Will their alarm monitoring service work with it on DHCP? If so, just set it to DHCP and let it be.

First reserve the IP address for the device at the end of the dhcp range.  then go into the device settings and change it from DHCP to static, and use the same address that you reserved.  it won't hurt anything, and will give you a backup in case the ISP resets the router.

All other devices are computers and phones so no need for static IP's on those devices. The reason for the static is for accessing the security system remotely, either through the software or using the smart phone app. Having the IP address setup as DHCP or Static doesn't matter to the monitoring station.

Not sure if that router has a DMZ setting for a port, but worth a look. Add your router and fix the issue.
I had wasted a bunch of time with ATT gateway box that had similar issues. Not worth it.

!!

Nonsense. The WAN side of the router has no concept of DHCP. An address is an address -- but you may need to open a port or set up a VPN for special services.

I encourage the customer to purchase a modem and skip the monthly rental fees. Payback is usually about a year. Then we can install a robust router and lock it down. Hopefully, my router will scare the cable guy and he'll leave it alone.

Although one can reserve an IP address for a device that needs a fixed address, I prefer placing fixed addresses outside of the DHCP range.

A mikrotik router can be had for 50 bucks. How many hours have already been wasted trying to use the crappy isp modem/router? Like Buzz said, 100 (or less)bucks to buy a modem, 50 bucks for a mikrotik router problems disappear. You are billing for all this time right?