Suppose I use ifconfig or some similar command to setup my network interface, and I assign to the mask parameter. Does it have a special meaning? What implications could this have?

2 Answers 2

5 generally means that the network interface isn't bound to any network at all, not even localhost requests will be acknowledged. If anything it's a way of the operating system to present the interface as available but inactive and un-routed.

  • Would ARP Request from the host owning said interface work? Jul 3, 2011 at 21:58
  • I don't think it would, it doesn't in Windows in any case. Jul 3, 2011 at 22:10

From ArsTechnica:

Think of a subnet mask as a filter for outbound traffic. The subnet mask helps the IP protocol decide which is local "LAN" traffic and what traffic needs to be forwarded to a router.

In effect a subnet mask would make it so all outbound traffic is 'local' and nothing will ever be forwarded to a router. This configuration should work fine if there is no router (aka Default Gateway) on the network.

If you do need to route you can always add static routes on the house by using the ROUTE ADD command (Windows, other OS's should have the same/similar command).

In this case (with a few edits for brevity). I can add a /0 or netmask to an interface, and it will work on loopback and link-local:

root@host:/tmp# ip addr add dev eth0
root@host:/tmp# ip addr show
root@host:/tmp# ip a s
    inet scope host lo
       valid_lft forever preferred_lft forever
    link/ether fe:dc:ba:98:76:54 brd ff:ff:ff:ff:ff:ff
    inet scope global eth0
       valid_lft forever preferred_lft forever
    link/ether 01:23:45:67:89:ab brd ff:ff:ff:ff:ff:ff
    inet brd scope global wlan0
       valid_lft forever preferred_lft forever
    inet6 fe80::226:c6ff:fe4b:e38e/64 scope link 
       valid_lft forever preferred_lft forever
root@host:/tmp# ping
PING ( 56(84) bytes of data.
64 bytes from icmp_seq=1 ttl=64 time=0.033 ms
64 bytes from icmp_seq=2 ttl=64 time=0.036 ms
--- ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 999ms
rtt min/avg/max/mdev = 0.033/0.034/0.036/0.006 ms

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