You may be set up behind a company proxy and need to clone an external url. If your admin is blocking port 9418, you won't be able to use the best option, which is the git protocol.
If you need to use http, you need to define your proxy as an environment variable with one of the following:
# no authentication required by your proxy export http_proxy=proxy.server:port # or # authentication required export http_proxy=username:password@proxy.server:port
Now you should be able to use:
git clone http://git.someurl.com/project.git