IP addressing ending .0 or .255

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There is much confusion on ip address ending with .0 or .255. In general, .0 would be considered as a network identifier and .255 would be a broadcasting address. That’s true. However, if you are fully understood about an IP subnetting theory, you know those IPs are usable and it can be assigned be on your network. REALLY! Sure you can!

Let’s see simple example below

From an IP block 192.168.100.0/24, network identifier would be 192.168.100.0 and broadcast would be 192.168.100.255. Sure it does. So, valid IP addresses will be 192.168.100.1~254. How about below an IP block?

192.168.0.0/16

It won’t be hard to guess what would be network identifier and broadcast address. As you guessed 192.168.0.0 will be network identifier and 192.168.255.255 will be broadcast address. Now, what is the valid IP address range then?

192.168.0.1~192.168.255.254

So, 192.168.10.0/16, 192.168.20.0/16, 192.168.99.255/16……. Those are all valid IP addresses. Don’t forget network mask, b/c without network mask it would be meaningless.

In rare case, DSL customers are complaining about their IP address .0 or .255 assigned automatically by DHCP server when they are having a reachability or connectivity issue. They thought the IP address was not valid IP address. Many DSL or cable ISP companies assign big IP block to their DHCP server and probably few of subscribers will get those IP addresses. Again, those IP address is are valid IP. Their problem is caused by something else.

If you are having reachability only certain sites, please check they have ip classless function on your destination network.

I hope this is informative for you.