Decode a partial final octet
02A mask width need not be a multiple of eight. Read enough destination octets, then remove every host bit.
The resulting route is 129.210.177.128/25 via the fictional next hop 192.0.2.1. It covers the network, not only host .132.
Source for this drawing: RFC 3442 · Read RFC source: Classless Route Option Format (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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Read ceil(width / 8) octets; install destination AND mask.
- Widths 0, 1–8, 9–16, 17–24, and 25–32 require 0, 1, 2, 3, and 4 destination octets respectively.
- The client MUST zero destination bits where the corresponding mask bit is zero before installing the route.
- For the RFC’s /25 example, 129.210.177.132 becomes 129.210.177.128 with mask 255.255.255.128.
The worked example deliberately uses the published nonzero host bits to show normalization. A /32 keeps all four octets; /0 keeps none. The helper rejects truncated input as an implementation guard, not a new RFC error rule.
Read the RFC section ↗