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I've been searching information concerning the meaning of scopes both in addresses and routes but I don't understand the documentation I've read so far. This is what I've been reading:

Is there other documentation I could read? Google for "iproute2 address scope" and similar was not helpful. What I thought an address scope would mean was how that address could be reached:

  • "host": only from localhost.
  • "link": only from other hosts directly attached to the network to which the corresponding device is attached to.
  • "global": "host" + "link".
  • Other scopes would be reserved for future usage.

I see no change in behavior if I change from "global" to "host", "link" or "3" the scope of the address my laptop is assigned through DHCP and the result was:

  • In all cases I could ping from my laptop to that address and to the Internet.
  • In all the cases I could ping from a virtual machine on the same laptop using a virtual NIC bridged to the same interface and with a manual network setup to that address.

Note that I'm only using IPv4.

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One effect of address scope is to determine when the address will be selected for outgoing connections. https://www.rfc-editor.org/rfc/rfc3484#section-4 (via man gai.conf).

Looking at the examples, the first given seems to be that it avoids using a link-local address when contacting a globally unique address. https://www.rfc-editor.org/rfc/rfc3484#section-10.1

To be honest I don't remember finding any other significance of scopes in Linux. Presumably there was some reason they're available on routes though. The address selection is a userspace thing, so I expect it's distinct from the routing.

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  • edit: I stared at the algorithm trying to think of an example. Eventually I remembered there's actually an example section. Suggested moral: worked examples make everything better :).
    – sourcejedi
    Commented Apr 15, 2016 at 12:00
  • RFC3484 stands for "Default Address Selection for Internet Protocol version 6 (IPv6)" and I'm working on IPv4. Commented Apr 15, 2016 at 12:04
  • Scopes are (originally) an IPv6 concept.
    – sourcejedi
    Commented Apr 15, 2016 at 12:08
  • update: actually the RFC says "The destination address selection algorithm operates on both IPv6 and IPv4 addresses. For this purpose, IPv4 addresses should be represented as IPv4-mapped addresses"
    – sourcejedi
    Commented Apr 15, 2016 at 12:14
  • Exactly, but I'm dealing with the source address. Commented Apr 15, 2016 at 13:01

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