The network command specifies which IP prefix the router should announce into BGP. You're usually announcing the address space you "own" as a single prefix (specifying address and mask).
For the prefix to be inserted in BGP table (and announced to other BGP routers), an exactly matching IP route has to be in the IP routing table (usually inserted with a static route to null 0 if you're originating the prefix on a single router).
If you're a Cisco partner, you can try this out for free - go to Partner e-learning connection, Lab central, find BGP labs and start the "Configuring basic BGP" lab.
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...