The modem will provide internet to the main computer, but not to the other computer or my xbox. I dont understand why

The router is provided by your internet provider some companys only have modems which means you would have to plug your Xbox into the modem whith a Ethernet cable call your provider to make sure the router has WiFi if so just go to settings on Xbox and conne

If its not a black xbox 360 slim than it doesnt have wirless so you either need a wireless adapter or ethernet cord that will hook up to the modem from the xbox.

if you keep getting disconnected then i would do internet test,if theirs no fault your end then i would contact your internet provider,because has long has its not xbox live which i dont think it his then i would ring your internet provider,also let us no more details has in does your internet go down or just xbox live? does your router or modem light go off every 20 mins then quickly come back on?

I am not that much clear whether you can change your Ip adress, but any queries regarding Ip-detais can be solved here, http://www.ip-details.com/

Do you have a router? Most cable/DSL modems have built in routers (more than one ethernet port). The ethernet cable looks like a large phone cable (it's called RJ-45). If so, you will need to powercycle your modem.

You can powercycle your cable modem by:

Unplug the power from your modem for 1 minute (no lights on the device). Plug it back in, and reboot both devices and they will both have IP address to get out to the internet.

Let me know how it goes or if you have anymore questions.

Is the main computer connected to the router or to the internet directly? If it is a cable modem for the internet, make sure you turn that off, as well as the router, and connect the cable modem to WAN on the router and put the modem on first, then the router. Connect the computers, xbox, etc to the router

if you dont have a router, you can either have the xbox or the pc conneted to the internet, not both. you have to have an eithernet cord or a usb 2.0 high speed usb chord that goes directly from the xbox to the modem. you cannot go from the modem to the computer then to the xbox.

Ok plug everything in and Reset the router and modem wait a few minutes and try again.
Verify that that DATA cable at the Xbox's location works with a PC could be a simple bad data jack or patch cord. Do you get a connection failure like this? MTU failure?

The MTU(Maximum Transmission Unit) failure when testing the xbox's connection can be do to a few things because Xbox live requires a minimum MTU setting of 1364. Sharing a slower internet connection with other PC's in the house can cause this error, The incorrect MTU setting in your routers programming can cause this error, The lack of firmware updates to your router can cause this error, an older modem from your service provider can cause this. Old or worn network cables and jacks and or RG(coax) cables if you have cable internet could cause this. Your Router or gateway could be incompatible with Xbox live.

Try a direct connection to the Xbox without a router and reset your modem before doing it. If your able to connect then you know its the router. If not you know its the modem or cables or jacks or even your Xbox if you havent tested it elsewhere.
You can tell your ISP to mail you the newest updated modem... sometimes they will have a guy just drop one off.

This is trial and error.

I would suggest that if you are xbox live unless you have fast internet dont use the internet for other things like your computer. Its most likely that there is not enough bandwidth for everything. If that doesnt work call you internet service provider and explain the problem to them and if they are a legitamate company they will help you out.

Shared connection with a modem and a router
1. Turn the modem off, and then turn it back on. Then turn the router power off, and then turn it back on. This is also known as power cycling the modem and the router.

Power cycle the modem first, and then power cycle the router. To power cycle these devices, you should follow your manufacturer’s specific instructions. Here are some general steps:
a. Remove the power supply from your device.
b. Leave the device unplugged for 15 to 30 seconds.
c. Reattach the power supply to the modem by following the device manufacturer’s instructions.
d. Wait several minutes for the device to start and to establish a connection with your broadband provider.
e. Verify that the modem shows the appropriate power and Internet connection indicator lights by following the manufacturer’s instructions.
2. If power cycling your modem and router does not resolve the problem, try connecting the Xbox 360 console to Xbox LIVE by using the modem only. This eliminates any configuration problems on the router. If this setup works without the router, there is probably a configuration issue with the router. See the manufacturer’s documentation that is included with the router or contact the technical support group for the router.
3. Verify that your router's Dynamic Host Configuration Protocol (DHCP) service has been enabled. DHCP enables the router to assign IP addresses to the computers and consoles on your network when you go online. If you need help with this, see the manufacturer’s documentation that is included with the router or contact the technical support group for the router.
4. If you manually entered an IP address, gateway, subnet mask and/or DNS servers in the Network Settings area of the Xbox Dashboard , verify that it they are correct. At this point, you can also set your IP settings to Automatic. To do this, follow these steps:
a. Move to the System area.
b. Select Network Settings.
c. Select Automatic.
5. If your ISP requires Point-to-Point Protocol over Ethernet (PPPoE) settings, try connecting to only the modem after you enter these settings in the Network Settings area of the Xbox Dashboard. Network Settings area of the Xbox Dashboard.

Note Some ISPs may require Point-to-Point Protocol over Ethernet (http://www.xbox.com/en-US/support/connecttolive/xbox360/resources/glossary.htm#ethernet) (PPPoE) settings, This means that you may need settings such as a username and password in order to gain access to your ISP’s network. If you are unsure whether you have to enter these settings, please contact your ISP support group. At this point, if you can connect directly to the modem, see your router documentation to configure the PPPoE settings correctly on the router.
6. If you are receiving an IP conflict error in the More Info section of the Xbox Dashboard, turn off all devices on the network. Try to connect to Xbox LIVE with only the modem and Xbox 360 console connected.

If this works, there may be a configuration issue on the router’s Dynamic Host Configuration Protocol (DHCP) server. DHCP enables the router to assign temporary IP addresses to the computers on your network when you go online. If you need help with understanding DHCP or the configuration, see the manufacturer’s documentation that is included with the router or contact the technical support group for the router.
Shared connection with a gateway device (combined modem and router)
1. Make sure that the gateway is turned on. A gateway integrates a high-speed Internet modem with a router.
2. Verify that there is network activity by looking for the sequence of blinking lights on your gateway that generally indicate Wide Area Network (WAN), Local Area Network (LAN), and connected power status.
Note These names will vary from model to model.
3. Turn the gateway device off, and then turn it back on. This is also known as power cycling the gateway. To power cycle this device, you should follow your manufacturer’s specific instructions. Here are some general steps:
a. Remove the power supply from your device.
b. Leave the device unplugged for 15 to 30 seconds.
c. Reattach the power supply to the gateway by following the device manufacturer’s instructions.
d. Wait for the device to start and to establish a connection with your broadband provider.
e. Verify that the gateway shows the appropriate power and Internet connection indicator lights by following the manufacturer’s instructions.
4. The IP address that you entered in to the Network Settings area of the Xbox Dashboard may be incorrect. Be sure that this address is correct, or set your IP settings to Automatic in the Xbox Dashboard.
5. If you are receiving an IP conflict error in the More Info section of the Xbox Dashboard, turn off all devices on the network. Try to connect to Xbox LIVE with only the modem and Xbox 360 console connected.

If this works, there may be a configuration issue on the router’s Dynamic Host Configuration Protocol (DHCP) server. DHCP enables the router to assign temporary IP addresses to the computers on your network when you go online. If you need help with understanding DHCP or the configuration, see the manufacturer’s documentation that is included with the router or contact the technical support group for the router.
Shared connection with Microsoft Windows Internet Connection Sharing
Shared connection with Microsoft Windows Internet Connection Sharing
1. Verify that you can surf the Internet from your PC. If you cannot surf the Internet, there is probably a problem with your Internet connection. Contact your ISP for support.

If you are using a USB modem to connect to the Internet, the modem will be listed in the "Dial-up" window as the connecter that you are sharing and that you have configured. To verify that Internet Connection Sharing configured it correctly, use the following steps:
a. Click Start, and then click Control Panel.
b. In Control Panel, double-click Network Connections.
c. Right-click your broadband connection, and then click Properties.
d. In the Properties dialog box, click the Advanced tab.
e. Click to select the Allow other users to connect through this computer’s internet connection check box.
f. Click OK.
Ethernet Properties
2. Power cycle the modem. If the modem does not have an on and off switch, unplug the modem from the power outlet. You cannot follow this step if you are using a USB modem. Instead, go to the next step. You must follow this step if you are switching the modem between the Xbox 360 console and a computer.
3. On the computer, click Start, click Run, type ncpa.cpl, and then click OK.
4. On the View menu, click Tiles.
5. If the connection is listed as Firewalled, the firewall may have to be configured. See your firewall documentation.
6. Enter manual settings in the Xbox 360 Dashboard for IP address, subnet mask, DNS address and gateway and test the connection: To edit the settings:
a. In the Xbox 360 dashboard, move to the System section.
b. Select Network Settings.
c. Select Edit Settings.
d. Enter the following:
• IP Address : 192.168.0.2
• Subnet mask : 255.255.255.0
• Default Gateway : 192.168.0.1
• DNS server (Primary): 192.168.0.1
• DNS server (Secondary): 192.168.0.1Xbox 360: Test Xbox LIVE connection fails at IP address