Subnet Masks

The Subnet Mask field of a RIP message contains the subnet mask that is applied to the IP address to set the nonhost portion of the address. If the subnet mask field in a RIP message contains a zero, then no subnet mask was included for the entry.

On an interface where a RIPv1 router may hear and operate on information in a RIPv2 routing entry, the following rules apply:

• Information internal to one network must never be advertised into another network.
• Information about a more specific subnet may not be advertised where RIPv1 routers would consider it a host route.
• Supernet routes (routes where a netmask is less specific than the natural network mask) must not be advertised where they could be misinterpreted by RIPv1 routers.

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