Most companies with somewhat restricted firewall rules typically enables only web traffic using port 80 or 443 (for SSL).
Git is communicating with Github using port 22 (or the default 9418), which in my own case was blocked after a while.
To keep “gittin” from work there are some options.
Work with pure http access (port 80)
git clone http://github.com/username/repository.gitThis will only give you read only access and is rather slow
Use githubs public server
ssh.github.com, which works via port 443
git clone ssh://email@example.com:443/username/repository.git
I use the second one, and it works great, haven’t seen any speed tolls so far.
Reference : http://returnbooleantrue.blogspot.com/2009/06/using-github-through-draconian-proxies.html