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I know that for addresses like e.g. 192.168.17.5 with 255.255.255.0 subnet mask, address 192.168.17.0 is the network id and 192.168.17.255 is the broadcast address and that valid hosts are from 192.168.17.1 to 192.168.17.254.

I understand why 192.168.17.255 can't be used as a host address since everything sent on this is a broadcast, but I can't understand what 192.168.17.0 is used for?

Is it just a reserved name of the network or is it used for something? Every thing I have read says you can't use it as a host address but not why? Is it just a convention?

Can someone please explain what it is used for and why it can't be a valid host address?

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duplicate of serverfault.com/questions/135267/… –  schöppi Oct 22 '10 at 10:38
    
See the answer with 10 upvotes there. Used for routing. –  Daniel Beck Oct 22 '10 at 10:40

1 Answer 1

up vote 2 down vote accepted

The network-address is for routing. Look at the binary representations for the ip address and the subnet mask. In the process of determining the route they are binary combined with AND. 1&0=0, 1&1=1, 0&0=0. The network part of the address remains unaffected, but the host part becomes all-zero. If you could use the .0 address for a host too, how would you different it from the net?

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I'm sorry, but that's the answer to another question. Not using subnet zero means if you subnet 10.0.0.0/8 further to 10.0.0.0/24 you shouldn't use addresses like 10.0.0.7 and 10.255.255.7. –  omnibrain Oct 24 '10 at 20:51

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