Before proceeding with troubleshooting, please check the Steam Server Status page to ensure the issue which you are encountering is not related to system-wide downtime. (If you encounter any timeout errors when attempting to access Steam sites, it is very likely that there is a system-wide issue and Steam will not be accessible until the issue is resolved)
Troubleshooting Network Connectivity Issues
Please follow the steps below to diagnose network connectivity issues within Steam:
Test Your Router
Before trying anything else:
Try bypassing your router by connecting your computer directly to your modem.
If the issue disappears after bypassing your router, please refer to the Using a Router with Steamtopic for instructions on configuring your router.
Note: running your router in DMZ mode is not supported by Steam and may result in connection issues.
Additionally, wireless connections are not recommended for online gaming. To ensure reliability, make sure you are using a wired connection to the internet while playing online.
Note: If you are installing Steam for the first time or Steam has recently been updated, a firewall may be preventing Steam from connecting to the Steam Network - even if Steam worked correctly before the update was applied.
All required ports to run the Steam application are listed here. It is possible that other games available on Steam will require further ports to be available.
Many university networks and proxies block required ports for Steam operation - please consult your network administrator to ensure the required ports are open if you are using a university network or a proxy. Ports required for Steam can not be re-mapped to HTTP or reconfigured to a custom port range.
Known Issues with Internet Service Providers (ISPs)
If you notice that your Steam connection only has issues during certain hours of the day (usually in the evenings between 5pm and 11pm), it is probable that your ISP is blocking or "shaping" your connection. Please test your connection at other hours of the day and contact your ISP directly if you believe this is causing the connection issue.
There are several ISPs which block ports required for the proper operation of Steam and Steam games:
012.net (Steam traffic blocked)
Bluewin (Firewall blocks Steam from provider side - must be disabled through provider's service portal)