What happens if the mask parameter isn't specified in a BGP network command?

• A. It advertises default routes
• B. It advertises the supernet
• C. It advertises all subnets
• D. The command only announces the classful network number

Answer: (D)

In BGP, the network command tells the router which prefixes to advertise, and the mask parameter specifies the exact prefix length to advertise. If you omit the mask, the router uses the default classful mask for that network address, so it only advertises the classful network number. For example, a Class A address like 10.x.x.x becomes 10.0.0.0/8, a Class B like 172.16.x.x becomes 172.16.0.0/16, and so on. This is why the correct outcome is that the command only announces the classful network number.

Default routes aren’t automatically advertised by omitting the mask; advertising a default or a specific broader/smaller set of subnets would require explicit configuration.