12

Let's say I am on a network 192.168.1.X and I use the below ping commands.

  1. ping 255.255.255.255
  2. ping 192.168.1.255

Why we are calling 1st one as limited and 2nd one as directed ?

In what category does the below will come ? ping 10.X.X.X

X is denoting the feasible value in an IP.

9

The first one reaches the NICs in the network segment, routers don't forward it, so it's limited. The second will reach all the hosts in the network if it's allowed by a router, so it's directed.

If you're talking about pinging 10.255.255.255, it's directed.

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7

Example may help to illuminate the subject:

A - B - C

A-B is 10.10.10.0/24
B-C is 10.10.20.0/23

If 10.10.10.42 wants to do say WOL (Wake-On-Lan) for 10.10.21.42, 10.10.10.42 on subnet A-B can send packet to directed broadcast 10.10.21.255, and B is happy to to forward the packet towards C, so each host in C LAN will receive it, including 10.10.21.42 machine which can then power itself on.

10.10.10.42 couldn't unicast it to 10.10.21.42, because the machine is not on and would not be visible in ARP table, so it would be just dropped after unsuccessful 'ARP WHO HAS' query.

If A tries to send it to 255.255.255.255, B will not forward it to C. 255.255.255.255 is only special case 'limited broadcast' address.

In this example 10.10.20.255 might not be a broadcast at all. So we cannot conclude if address is directed broadcast address without knowing the mask.

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  • Hi, good answer. In the last sentence, 10.10.20.255 is definitely a broadcast address since we know that 10.10.20.0 is the Network Id. I believe you meant to say that "10.10.21.255 may not be a broadcast at all". – Vishnu Vivek Nov 29 '15 at 18:39

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