How can I setup a subnetwork using DDWRT and a 2Wire?

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http://serverfault.com – Ok, I'm having a bit of trouble getting this setup. I'm not really sure what direction I need to be looking in. I am on ATT UVerse and I have to use their horrible 2Wire Modem/Router. But with this router, I have very limited control over my network and my wifi range is too small for my house. But, I have a hardwire ran to my TV which I have hooked into a switch and split the line to my PS3. The problem I'm having, is I don't know how to get the Buffalo DD-WRT setup as a subnetwork so that the PS3 can find my Media Server. Here is a diagram of what I'm trying to accomplish: Blue is ethe (HowTos)