ip subnet-zero

What is IP Subnet Zero?

Before we talk about the command, let’s ask ourselves, “In the first place, what is a zero subnet?” Under old IP subnetting rules, the all 0’s subnet was reserved for the network, and the all 1’s subnet was reserved for the broadcast. Over time, engineers found that the all 0’s subnet wasn’t really used and, if it could be handed out as a useable network, many IP addresses could be changed.

An example of an IP address that is using a zero subnet is with a subnet mask of This IP address may look pretty weird to you. Some people may even try to argue that it is an invalid IP address because there is a 0 in third octet. However, today, this IP address is perfectly legal when it comes to subnetting. Thus, if I had an IP address of with a subnet mask and wanted to subnet it, I could actually get 255 valid networks out of it by using the 0 subnet. In other words, I could have networks ranging from 10.1.{0-254}.X where the X represents hosts 1-254. This gives me room for networks 0-254, or 255 total networks, by using the 0 subnet.

Thanks to Petri

* Another good explanation by The Packet University

* And more in CISCO itself

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