From https://stackoverflow.com/a/40189197/156458

Blocking makes no sense. In IPv4 it is never routed.

What does "routing" a IP address mean?

Why is it the reason that blocking does not make sense?


  • 2
    You route packets, not addresses. The packets are routed based on the destination address in the packet header. The address (actually, any address in the network) is not allowed to be a destination address, so explicitly blocking that address doesn't really make sense.
    – Ron Maupin
    Commented Mar 23, 2019 at 18:57
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can post and accept your own answer.
    – Ron Maupin
    Commented Jan 4, 2021 at 23:36

1 Answer 1


"Routing" means forwarding a packet based on its network-layer destination address, usually IPv4 or IPv6.

Routing is done by comparing the destination address to the entries in the local routing table and using the best (=longest prefix) match. The entry contains the next-hop gateway or the interface that the packet is then sent to/out of. is the default route - because the prefix has zero length it matches any address. However, it is only selected when no other routing entry matches.

As IP address, generally cannot be used (except as source address when configuring an interface, such as with DHCP).

  • Thanks. Does (as an IP address) have two completely different meanings? (1) A process specifies its own unknown address to another process not in the same host, e.g. the DHCP example, and (2) a process specifies that it is listening on all the addresses on the local host, e.g. netstat shows mysql server is listening at ::mysql (unix.stackexchange.com/q/508009/674)?
    – Tim
    Commented Mar 23, 2019 at 16:42
  • In "Blocking makes no sense. In IPv4 it is never routed", does mean a IP address? Why is it never routed?
    – Tim
    Commented Mar 23, 2019 at 16:45
  • Yes, is also used when specifying "all local IP addresses" when allocating a BSD-style socket - however, this is host-specific and thus off-topic here.
    – Zac67
    Commented Mar 23, 2019 at 16:46
  • You can't use as destination address - so it's never routed.
    – Zac67
    Commented Mar 23, 2019 at 16:56
  • Thanks. When a server uses to indicate it is listening at all the IP adresses of local host, is used as source address, destination address, or neither?
    – Tim
    Commented Mar 23, 2019 at 21:46

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