You need to find an offsite slave name server for one or more of your zones.
Ask your ISP whether slave name service is included in the package of services they offer you. If not, ask whether they offer the service for an additional cost.
If your ISP is unable or unwilling to handle slave name service for your zones, there are alternatives available. Nominum offers free slave name service for noncommercial use at http://www.secondary.com/. Nominum also operates a commercial slave-hosting service, called GNS (Global Name Service), at http://www.nominum.com/services/gns/, as does UltraDNS, at http://www.secondarydns.com/.
Finally, you may want to consider trading slave name service with another organization, say a partner company or a university you have a close relationship with.
Far too many organizations have a single point of failure in their DNS infrastructure: all of their authoritative name servers on the Internet are located on the same subnet, behind a single router. While many of these organizations would like to have an external slave name server, they're often scared off by the perceived expense of such a solution. But remember that you probably don't need to set up your very own offsite slave; many companies run slave name servers that are authoritative for thousands of zones, and they'd be willing to load your zone, too, for a reasonable sum of money.
3.9.4 See Also
Section 1.17 for instructions on configuring a name server as a slave for a zone.
BIND Name Server Configuration
BIND Name Server Operations
Delegation and Registration
Interoperability and Upgrading
Resolvers and Programming
Logging and Troubleshooting