At some point or another, you may find it necessary to bridge a modem in order to access the internal network from the outside. If this is not done, you will have issues due to security settings on the modem. When you bridge a device you make it a level 2 device from a level 3 device. Most modems nowadays do not do a true "Bridge Mode" but more of a "IP Passthrough Mode". Never put a modem and router on the same IP subnet. Packets will not know which gateway to go through and will get stuck in a loop. Also note, a lot of ISP's use PPPoE logins. That means they use a U/P to get the internet used to authenticate.
To Resolve: Edit
1. Get on the only workstation with the internet if they just installed a new modem. If not, take the cord from the ethernet port on the modem to the workstation directly.
2. Make sure the workstation is on DHCP (Run - npca.cpl - set everything to automatic on the IPv4 settings).
3. Get on another workstation and note what the default gateway is. This should be router's IP address. You do this to make sure that you set the modem's IP to something different.
4. On the computer that is connected directly to the modem, go to the modem's configuration page (by typing the IP address of the default gateway in browser).
5. Change it's "Private LAN Subnet" or the "Modem's IP Address" to anything but the internal network. It's recommend the same setup as the rest of the network but a different subnet. Ex: Network = 192.168.1.1 / Modem = 192.168.0.1.
6. Make sure the "DHCP Settings" matches the new subnet. Ex: 192.168.0.1 will have DHCP as 192.168.0.61-192.168.0.253 or something like that.
7. Apply changes and try to enter the new IP of the modem in an internet browser. This is to make sure the settings took effect.
8. Take the cord going from the "ethernet port on the modem" and plug it in on the "WAN port on the network's main router". The modem should not have any cords going out of it to another device except the network's router.
9. Then, take an ethernet cable from the "LAN port on the router" (NOTE: Any LAN Port will work. Most router's have numbered ports 1-4, any of these will work) and connect it to the network "Switch". This device will distribute internal network access and internet access to the rest of the network. All computers on the network should go through this switch.
10. Plug the ethernet cables to all workstations that go to the switch and power cycle the modem, router, and switch and power them back up in that order. Test, you should have internet on all machines now.
11. If you don't have internet, go to the routers IP address in a web browser (if you have to, connect one workstation directly to one of the LAN ports on the router and make sure it's set to DHCP).
a. Once logged in to the router, navigate to "network - primary connection - edit" or something similar.
b. Change the options for "Port" to "WAN", "Connection Type" needs to be "LAN", and "Obtain IP Automatically" needs to be checked.
c. If the modem uses PPPoE, you need to enter the username and password from the ISP.