Subnetting and IP addressing can be difficult if you don't understand network theory. This tip, reposted courtesy of SearchNetworking.com, explains the network theory behind IP addressing so that you can understand why, in most cases, IP addresses shouldn't end in .0 or.255.
There's much confusion about IP addresses ending in .0 or .255, and this is clearly the result of misunderstanding the rules of subnetting -- a topic most newcomers dread!
In the majority of cases you can't end an IP address in .0 or .255 (but there are some exceptions!). Rather than give you a range of numbers to memorize, I'm going to give you a bit of theory as to why you can't use certain addresses to help you truly understand the reason.
As you would already know, every network uses some type of addressing scheme. This is required in order for the computers to communicate with each other. The ranges used can vary and depend on the administrator's preference and the network's requirements. As such, most select the good old 192.168.0.0 address range, while some prefer shorter addresses like 10.0.0.0. These addresses are what we call 'logical' addresses, meaning that they are based purely on software (the operating system).
One of the very basic rules in IP networks is that every logical network, no matter how small or big it is, must have a network identifier address and a network broadcast address. To make things simple, the engineers that designed all of the above though it would be a great idea to use the first and last addresses of each logical network to do their job!
So, as you've already correctly guessed, networks ending in .0 or .255 are not usable… in most cases! The reason for this is because we tend to allocate full Class C type addresses to our networks, making them obviously very simple for us to administer.
A standard Class C network consists of 256 addresses (0 to 255 inclusive), of which one is the network address (.0) and the other is the network broadcast address (.255).
To help make this clear, if we selected network address 192.168.0.0/255.255.255.0 for our office, then we can say that:
- The network identifier would be 192.168.0.0
- The broadcast address would be 192.168.0.255
- Valid (or usuable) IP addresses would be 192.168.0.1 to 192.168.0.254
In the above example, addresses .0 & .255 are not usable, but in the one to follow, they are.
Consider a huge office with more than 15,000 workstations. You decide that you want all these hosts in one logical network, so a Class A network address is not able to cover your needs. A Class B gives you a total of 65,535 IP addresses -- a really big network which you would surely want to break down, but for the sake of this example, we won't.
If we consider 172.16.0.0 / 255.255.0.0 as the preferred choice, then the network characteristics are as follows:
- Network identifier: 172.16.0.0
- Network Broadcast: 172.16.255.255
- Valid (or usable) IP addresses: 172.16.0.1 to 172.16.255.254
If you take a closer look at the valid addresses, you'll notice that IP addresses 172.16.255.0 and 172.16.0.255 can be used!
The above example is the reason I didn't want you to memorize that all IP addresses ending in .0 or .255 cannot be assigned!
The trick to understanding all the above is of course the network theory. If you would like to read more about subnetting, IP classes, etc., then I highly advise you to visit www.Firewall.cx and check out the tutorial to subnetting and IP addressing.
About the author
Chris Partsenidis, founder and senior editor of Firewall.cx, has over six years of experience as a senior network engineer, network administrator and as network consultant for companies in Australia and New Zealand. He is currently a Network Engineer with ALUMIL Milonas S.A, one of Europe's largest aluminum extrusion companies. In that position, Chris administers the network infrastructure, maintains and manages their VPN connections and network servers. Chris has a bachelor's degree in electrical technology. Chris also holds the following certifications: CNA for Netware 3,4,5, CCNA, MCP, DCE (D-link Certified Engineer), LCP (Linux Certified Professional), Network+ and A+.
Chris is the founder and senior editor of Firewall.cx -- a Web site he created to help the IT community benefit from his networking knowledge. Today, Firewall.cx has become a respected Web with over 450,000 page views per month.
This tip originally appeared on SearchNetworking.com.