Wi−Fi doesn't have valid IP configuration (Windows 8(.1))

Maybe all you need to do in order to be able to reconnect to the Internet is to restart the computer. Restarting the computer will restart every software (and every driver) running on it.

Sometimes, resetting the Transmission Control Protocol/Internet Protocol (which is a suite of communication protocols used to interconnect network devices on the Internet - read more on Searchnetworking) is all you need to be able to browse the net.

To reset the TCP/IP, follow the steps below:

1. Go to the start menu and type CMD, or command prompt;
2. Once it appears, right-click on it and select Run as Administrator;
   * Enter the administrator's password, if prompted
3. Once the Command Prompt opens, type the following commands in the same order as they are written here, and hit Enter once you write each of them (i.e. write the first one and hit Enter and write the second one and hit Enter):
   
   netsh winsock reset
   netsh int ip reset
   
   
4. Close the command prompt and restart the computer.
   
   

Note: If you use a static IP address you’ll have to set it again once your computer turns on. If you don't know how or whether your IP address is static or dynamic, contact the Internet provider you are subscribed to.

Maybe disabling and enabling the Wi-Fi card, i.e. restarting the Wi-Fi card, is all you need to do in order to fix this issue. 

Check out the don't know how to enable/disable the Wi−Fi guide, if you are not sure how to disable the Wi-Fi card.

Maybe the router is responsible for this issue. Try turning it off and back on after a few minutes.

By changing the router's SSID, i.e. the wireless network's name, and password, you will "deceive" the computer that a new Wi-Fi network is available and you should be able to connect to it and browse the net, once you enter the password.

To change the SSID and password, you need to connect your computer with your router using an Ethernet cable, access the router (check out the don't know how to access my router guide, if you are not sure how), go to the Wireless section and change the wireless network's name (SSID) and password.

To change the password of a TP-Link router, feel free to check the video below:

Maybe setting the channel width to Auto will resolve the issue. If you are not sure how to change the channel width, follow the steps below:

1. Go to the start menu and type view network connections and click on it once it appears in the search results;
2. Right-click the network connection you are trying to connect to and select Properties;
3. Click the Configure button and go to the Advanced tab;
4. Locate 802.11n channel width for band 2.4 and set it to Auto.
5. Click OK to save the changes you made.

According to some users, increasing the number of DHCP users should solve this problem. You can increase the number of users via the router's UI (User Interface). Since the router's UI differs from one router to another, I suggest you to consult your router's manual, or their website, on how to increase the number of DHCP users.

Some users report that increasing the number of maximum wireless users which can connect to the router simultaneously resolves this issue. By default, the maximum number of users that can connect simultaneously is 10-15, depending on the router.

To change the maximum number of users, you need to access the router's UI (User Interface), which is not the same on all routers. Consult the router's manual, or their website, on where to find this option and how to change the number of users.

The "Windows Network Diagnostics" troubleshooter is designed to resolve a wide range of network problems that might occur. To run it, right-click the Wi-Fi icon in the notification area (by default the bottom right-hand corner of the screen), select Troubleshoot problems and follow the instructions.

Maybe the antivirus software (the firewall) is the root cause of this problem. To test if the antivirus itself is the culprit, disable the antivirus software and restart the computer. 

If uninstalling the antivirus software resolved the issue, download and install the latest version of the antivirus software, from the antivirus website.

Maybe a startup program (i.e. a program which is set to start automatically when you turn on the computer) is the culprit. To check if this is the case, boot the computer into Safe mode with Networking. If you are able to connect to the Wi-Fi network, one of the startup programs is the culprit.

In this case, you need to disable the startup programs, usually a program you installed, in order to find the culprit, and after you find it to update the program or reinstall it.

If you are not sure how to disable startup programs, check out the don't know how to disable startup programs guide.

Maybe a problem with the network is the reason for this issue. Releasing and renewing the IP address should help you resolve this problem. Follow the steps below in order to release and renew the IP address:

1. Go to the start menu and type either command prompt or cmd;
2. Once it appears in the search results, right-click on it and select Run as Administrator;
3. Once the Command Prompt opens, type the following commands in the same order as they are written here, and hit Enter once you write each of them (i.e. write the first one and hit Enter and write the second one and hit Enter) :
   
   ipconfig /release
   ipconfig /renew
   
   
4. Close the Command Prompt
   
   

Feel free to watch the video below, if you don't feel confident enough to use the command prompt:

According to some users, flushing the DNS helped them resolve this issue.

Check out the don't know how to flush the DNS guide to learn how to flush the DNS.

If none of the other solutions worked, it is possible that some of the necessary wireless network related services are disabled. To enable them, follow the steps below:

1. Go to the start menu and type Services;
   
   
2. Locate all of the following services and ensure that they are enabled, by double-clicking every one of them clicking Start, under Service Status:
   
   DHCP Client
   Network Connected Devices Auto-Setup
   Network Connection Broker
   Network Connections
   Network Connectivity Assistant
   Network List Service
   Network Location Awareness
   Network Setup Service
   Network Store Interface Service
   WLAN AutoConfig
   
   
3. Change their Startup Type to Automatic;

1. Move your cursor to the bottom right corner of the screen and click Settings;
2. Click the Network icon;
3. Right-click the network you want to forget and click on Forget this network;
4. Reconnect to the network by entering its password.