Using .0 and .255 as Host IP's | Network Direction
Using .0 and .255 as Host IP’s | Network Direction
Many years ago a colleague of mine asked if we could use .0 and .255 IP addresses for hosts. ‘Of course not!’ I replied, ‘they’re special IPs that are always reserved for the network ID and the broadcast address’.
I was studying for my CCNA at the time, and I was feeling pretty confident in my answer. The only problem is… I was completely wrong. In fact, there are three different cases where this is quite possible, and I’m going to show you each of them.
Some things are obvious in hindsight, and this is one of those times. Usually, the first and last IP addresses in a subnet are reserved to be the network ID and the broadcast IP. No surprises there. And often, we use a /24 network as they’re nice and easy. So, in those cases, .0 is the network ID, and .255 is the broadcast IP.
But what if we’re not using a /24 network? What if we’re using a /23? For example, we might be using 192.168.10.0 /23. The network ID then, is 192.168.10.0, and the broadcast IP is 192.168.11.255. These are the first and last addresses in the subnet.
So what about 192.168.10.255 and 192.168.11.0? Are they special? No! They’re just regular host IP’s.
There’s another way this can happen. Have you ever heard of a /31 subnet mask? This allows for exactly two IP addresses. But if the first and last address are reserved, how can this work? A /31 network is an exception to this rule. There is no network ID and no broadcast IP.
This type of addressing is only used on a point to point network, where we don’t really need broadcasts and host ID’s.
The last one’s a bit of a cheat. On most network devices, we can configure loopback interfaces. These can have a /32 address assigned to them, which can be any valid IP, including .0 or .255.
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Overview of this video:
0:00 Introduction
0:34 The /23 Network
2:11 The /31 Network
3:01 Loopback interfaces
LET’S CONNECT
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