Think of Blog Networks as niches you are optimizing your sites for. For example if you have a money site in the health industry, create a “Health Network”. All blogs in that network will get a unique IP. If you have another money site in the blogging niche, create a “Blogging Network”. Some IPs between these two networks might be the same but that doesn’t matter because you’ll be linking to other money sites.
Blog Networks are not just simple tags or categories. Blogs are deployed to different servers based on their set network. Changing networks means moving blogs to different servers, so make sure you do it right from the beginning.
You can manage networks on the Settings page.