Tried xbox live support, but they just give me the run of the mill answers.  This one's tough, so I'm looking for some out-of-box options.  Thanks for reading.

I can play games on XBox Live, but I'm unable to connect to the XBox Live Marketplace.  Not a huge problem, but I'm also unable to get game updates and, more importanly, update the XBox Live service when there's a change.  Without the ability to update XBox Live, I lose my ability to play online.  The challenge is that I'm on a military base and behind the network adminstrator filters.  The admin is more than willing to open the filter up for Marketplace and update access, but he needs more information on what to open up (he's willing to help if I give him an easy solution, but he's providing admin coverage for thousands of users, so he's not going to spend a lot of time troubleshooting the issue himself).  The problem is more than likely the network software filters and here's why I think that:  on two occassions, the network department dropped the filters to perform maintenance and analysis.  Both times, the Marketplace and game/XBox Live updates working perfectly.  Combined with the fact that everything works fine when I'm out in the community using the same ISP on my same hardware and I think I can eliminate all problems aside from the filters.  Admin has asked for a URL (as most of the filtering is website specific apparently), but as far as I know, XBox Live Marketplace (and game/service updates) don't reside at a traditional URL or at least one I can find.  Any big brains out there have any idea on what filter modifications I can ask the administrator to modify in order gain access to the Marketplace? 

Thanks again for looking.

After reading the wall of text, I give you http://support.microsoft.com/kb/908874 now this can either be a direct problem, or a NAT/MTU problem within the networking where you currently are.

When trying to access anything on the marketplace do you get any error messages at all?
Have you tried running the Xbox live Connection test on the dashboard?

This is Asuming you already have the NXE update. http://support.xbox.com/support/en/us/nxe/xboxlive/default.aspx
http://support.xbox.com/support/en/us/nxe/xboxlive/troubleshooting/NetworkTest.aspx

Sorry don't have that many answers, but those pages should help you along the way, because I would end up paragraphing more questions in one go than you can reply to. Take a look on the second link an see if you can find any extra information that is todo with your current situation.

If you need anymore help do post again.

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"Information is not knowledge"
MW2: Veteran completion time: 6hours 23mins Spec Ops: 69/69

It's not a NAT/MTU problem, unless the Marketplace requires different peformance than the gaming portion of the service.  I took all my stuff downtown yesterday (including my router) and updated my box just fine (same ISP, just outside my network filters).  As usual, I'm back online today (at my place) and playing just fine.  There's no problem with the networking connection page.  It connects fine, including an 'open' NAT, and I play fine.  When I log in, the system tells me that some content is unavailable and the only tabs available are "My XBox" and "Friends", but it's nothing new as I've been going through this over the last 4 months.

If it's in a Microsoft support thread, it's not going to fix my problem (note the four months of troubleshooting comment).

It comes down to the network blocking Marketplace.  What's different about access to the Marketplace than access to the gaming servers?

If you are saying 4 months then I would roughly say that could of been the last time you updated the console, in which case you could have corrupt data in your cache causing the xbox to ignore the marketplace.

Simply follow the solution here: http://support.xbox.com/support/en/us/nxe/kb.aspx?ID=942630&lcid=1033&category=xboxlive

If this does nothing, then the only other option is portforwarding or something on the network is filtering all connections and blocking packets. The marketplace is basically realtime, connected to the LIVE service, where games online are all P2P based, but because you can play games, then your incoming connections are going work, because you have outbound traffic.

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"Information is not knowledge"
MW2: Veteran completion time: 6hours 23mins Spec Ops: 69/69

Nope.  4 months is how long I've been on the network.  As mentioned, I took my xbox down to a friend's place (last night) and updated it (XBox Live required an update).  I took my own router and my own xbox and was on the same ISP, just out from under the network.  Port forwarding isn't an option since I don't manage the network, but the dropping of a filter could be if I could help the admin determine what filter would be affecting the Marketplace, but not the gaming service.  Any other ideas anyone?