There is a lot of information missing from your question, so we'll go for a basic fault-seeking exercise.
There are two places where you need to look at the network:
- the VirtualBox environment
- the broadband access
First have a look at the VirtualBox environment. The easiest would be to put it in bridged mode. If you have another system on your network between the VirtualBox-host and the broadband provider, you should be able to check if your CentOS is accessible from outside your VirtualBox-host.
If it is not accessible, try the other way around: can you access the "other" system from your CentOS? A ping should be sufficient. Is the CentOS in the same subnet as the 'outside' network? Make sure that the host and the CenOS have a different IP address. Things like that.
Once your able to access the CentOS from your local 'outside' network, look at the broadband access. What type is it? If it is NAT (as often is the case in home access stuff) is port forwarding configured correctly (i.e. to the CentOS, not to the VirtualBox host)?
You should now have sufficient pointers to work on.