http://support.google.com/talk/bin/answer.py?hl=en&answer=27930

Required ports to use Google Talk

If you're having trouble connecting to Google Talk through your firewall or proxy, it may be because some necessary ports are blocked. In order to connect to Google Talk and start sending IMs, you'll need to enable TCP connections to talk.google.com on port 5222, or on port 443.
If you'd like to make calls or transfer files through Google Talk, you need to:

• Enable UDP connections to anywhere on any port; or
• Enable TCP connections to anywhere on port 443.

If you're behind a public or corporate network that is blocking these ports or protocols, please contact your local network administrator for further instructions.

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