If this is a corporate system, then work with your manager and with corporate IT to get the blocks removed for these systems; you have a legitimate use and legitimate need here, and IT is blocking that.
If IT is unable or unwilling to whitelist or enable that access for individual OS X systems, then setting up an OS X Server system with Software Update or a Linux or other system with Reposado might be an option. (For those OS X versions that can acquire updates locally.) Enanble the OS X Server or Reposado server for firewall access.
If this is a personal system (and assuming IT policy forbids connecting personal systems on the corporate network), then you'll need to do the upgrade elsewhere; at a coffee shop, library or Apple store.
Based on some other postings around the forums — I have not tried either of these links! — the downloads are (or were) apparently available via Apple's content delivery servers at the URLs:
There are cases where these URLs get spaces in them, so check for that if you get download errors.
Do work with IT if these are corporate systems, as IT is blocking vendor-provided security updates, and that sort of thing doesn't look very good on the IT director's and CIO's resumes.
I'm going to ask a "Stupid Question". Does your company have a "Proxy Server" and have you entered the "Proxy" information in your System Preferences -> Network -> Advanced -> Proxy ?
NOTE: If you do enter the company proxy information and this system is taken outside of work, then you will need to either setup a 2nd Network "Location" (one for work and one for not work) or remove the proxy informaiton when the system is outside of work. A separate Network Location is the preferred approach as you can then easily switch between locations using the Apple menu -> Location entry to quickly switch, or even get a utility such as ControlPlane to automatically switch based on where the Mac is currently connecting to the network.
But as MrHoffman says, if this is a corporate Mac, they need to allow updates or they may create a security risk if you are not allowed to get the security updates. That would look bad on their department. If it is your own Mac, they may or may not care.
swcdn.apple.com is blocked.
At the moment the IT department just says they only support windows. It is a Corporate machine but it is not supported.
We have a proxy and it is configured properly, there is just a firewall/filter that specifically blockes blacklisted sites, they appear to have swcdn.apple.com as a blacklisted site.
Unfortunately it is causing exactly the opposite effect they are looking for as some Mac users are starting to look on third party sites for updates, which is much less secure than allowing official Apple updates !!!
I will follow the link at home and bring the download in tomorrow.
Thanks for your suggestions guys.