Reach another subnet on the same link
04One link can carry several IP subnets. A zero router address can represent direct delivery to another subnet on that same link.
In a separate unsupported-stack case, that zero-router route is ignored. The example nonzero-router route is still installed.
Source for this drawing: RFC 3442 · Read RFC source: Local Subnet Routes (unnumbered; page 4) ↗
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A message needs an address.
Think of a packet as a small addressed envelope. Devices pass it onward; transport protocols decide what delivery promises an application gets.
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Related RFCs and changes
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0.0.0.0 is a conditional on-link instruction, not a router to contact.
- For other subnets on the client’s link, a server MAY be configured to send a route whose router address is 0.0.0.0.
- A client whose underlying TCP/IP stack cannot support this capability MUST ignore those zero-router routes. It can still install other 121 routes.
- This zero-router behavior applies only to Classless Static Routes; it does not apply to Router option 3 or Static Routes option 33.
The RFC’s private example has 10.0.0.0/24, 192.168.0.0/24, and 10.0.21.0/24 on a single link. Different subnet numbers do not always mean different physical links.
Read the RFC section ↗