Most networking questions are answered by a search. A smaller set needs an authoritative answer, either because the detail matters or because the search results are wrong often enough to be untrustworthy.
These are the sources worth going to directly rather than through a search engine.
Specifications and registries
| Source | Answers |
|---|---|
| rfc-editor.org | What the standard actually says, rather than a summary of it |
| iana.org | Port assignments, protocol numbers, address space allocations |
| datatracker.ietf.org | Draft status, and whether a standard was superseded |
The last one is the underused one. A great deal of published guidance references documents that were obsoleted years ago, and checking the status takes seconds.
Address and routing data
| Source | Answers |
|---|---|
| bgp.he.net | Who originates a prefix, and the autonomous system relationships |
| stat.ripe.net | Historical routing and address data for a prefix |
| peeringdb.com | Where a network is present and how to reach its operators |
| whois databases | Registered holder of an address block or autonomous system |
These matter during an incident involving somebody else's network, which is when the ordinary tools stop at the edge of your own control. Knowing who originates a prefix and where they peer is frequently the difference between escalating usefully and escalating blindly.
Measurement from outside
| Source | Answers |
|---|---|
| Public looking glasses | What a route looks like from a provider's perspective |
| atlas.ripe.net | Reachability and path from many independent vantage points |
| Team Cymru bogon lists | Address space that should never appear in routing |
The value of an external vantage point is that it answers a question your own network cannot. If a prefix is reachable from inside and not from elsewhere, the fault is in advertisement or filtering rather than in anything visible locally.
How to use them well
The pattern worth building is to reach for the registry before the search result when the question concerns an assignment, and for the external vantage point before the local one when the question concerns reachability.
Both habits save time in the situations where time matters, which is the only reason to keep a list like this at all.
Note: a looking glass shows one provider's view, not the internet's. Confirm from several before concluding that a prefix is missing globally, because a single missing view is far more often a policy at that provider than a withdrawal at the source.