If you work with networks, odds are pretty good that you're familiar with Cisco IP subnetting. But that doesn't mean you know all there is to know. It never hurts to refresh your subnetting skills—particularly if you're planning on taking a Cisco exam soon.
IP subnetting isn't some obscure, esoteric topic that only the network administration "gurus" can understand. In fact, you should be abl= e to get a firm grasp on subnetting in about an hour.
However, just like almost anything else, you won't be able to understand subnetting unless you use it—and practice. Learning subnetting takes = some repetition. And once you do learn it, you'll quickly forget it if you don't practice it periodically.
Subnetting means breaking a large network into smaller networks. You can accomplish this by changing the subnet mask.
Every computer needs three components in order to communicate both on its network and outside its network (e.g., to the Internet).
Routers function similarly to computers; every interface has an IP addre= ss and a subnet mask. While routers can also have default gateways, they don't always have them.
In addition, every interface on a router must be on a different network.= In fact, the router won't let you place two interfaces on the same network.
Published in 1990, RFC 1166 specified five different classes of IP addre= sses:
However, these fixed classes and subnet masks didn't fit the growth requirements of the Internet, and the industry abandoned them in favor of Classless Inter-Domain Routing (CIDR) in 1995 (RFC 1817). CIDR uses variable length subnet masks (VLSM) (RFC 1518 and RFC 1519). That means the classes above, with their fixed length subnet masks (FLSM), = were no longer applicable.
VLSM means that the subnet mask can dynamically change based on how many networks or hosts you need. So, while it's important to know the default (f= ixed length) subnet masks for testing purposes, a subnet mask for an IP that beg= ins with 192 may or may not be 255.255.255.0.
Subnetting is taking the original subnet mask and adding bits to it to create more networks. In other words, if you convert a subnet mask into bin= ary form and add more 1s, you are subnetting that network. The more 1s that are present in the subnet mask means the more networks with a fewer number of hosts. The more 0s that are present in the subnet mask means the fewer netw= orks and the more hosts you have.
In other words, the subnet mask tells routers and computers which portio= n of the IP address is for the network and which is for the host. If you slide t= he line between network and host to the left, you create fewer networks and mo= re hosts. If you slide the line to the right, you create more networks and few= er hosts.
You can write out subnet masks in two different ways: dotted decimal notation and slash notation. The dotted decimal notation method is more than likely what you're familiar with. This involves writing the numbers and separating them by periods (i.e., dots). For example, you would write a Cla= ss C default subnet mask as 255.255.255.0.
To use the slash notation method, you convert the subnet mask into binary form, writing the number of 1s in the subnet mask instead. For example, in binary form, 255.255.255.0 would be 11111111 11111111 11111111 00000000.
In slash notation, this "translates" to /24—in other wor= ds, the number of 1s. So, you would write your IP network with its subnet mask = like this: 192.168.1.0/24. While the slash notation method may seem more complicated, keep in mind that it makes diagramming and note taking easier.=
You can find a wealth of information on the Internet to learn more about subnetting. Here are some resources I recommend:
Stay tuned: Next time, I'll tell you five more things you need to know a= bout Cisco IP subnetting, including various subnetting formulas, where to find subnetting calculators, what you need to know about subnetting for the CCNA exam, and more.
Want to learn more about router and switch management? Automatically sign up for our free Cisco Routers and Switches newsletter, delivered e= ach Friday!
David Davis has worked in the IT industry for 12 years and holds seve= ral certifications, including CCIE, MCSE+I, CISSP, CCNA, CCDA, and CCNP. He currently manages a group of systems/network administrators for a privately owned retail company and performs networking/systems consulting on a part-t= ime basis.
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IP subnetting isn't some obscure topic that only network administration "gurus" can understand. In fact, if you work with networks, odds = are pretty good that you're already somewhat familiar with Cisco IP subnetting.=
But it never hurts to refresh your subnetting skills—particularly = if you're planning on taking a Cisco exam soon. Last time, I told you five things you should know about Cisco IP subnetting, including what subnet= ting is, how to write out subnet masks, and more. Now, let's look at five more things you should know about subnetting.
To review, subnetting involves taking a single network that has a large number of hosts and breaking it into multiple networks each with a smaller = number of hosts. That's why we call it subnetting—because we're creating = subnetworks. Let's look at two formulas you need to know when subnetting a network.
Networks formula
This formula is 2x, where x is the number of 1s added to =
the
subnet mask from the previous or default subnet mask when converted to bina=
ry.
For example, a subnet mask of 255.255.255.252 converted to binary is 11111111 11111111 11111111 11111100. Let's assume that this is from a cl= assful network (i.e., a network that uses the five different classes of IP address= es), and it started out as a Class C subnet mask (255.255.255.0). To arrive at t= he new subnet mask (255.255.255.252), we added six 1s to the subnet mask, which had a default number of 24 1s.
So, 26 =3D 64, which means that changing the subnet mask from= the default /24 (255.255.255.0) mask to a /30 (255.255.255.252) subnet mask wou= ld create 64 new networks.
Hosts formula
This formula is 2y-2, where y is the number of 0s in the
subnet mask when converted to binary.
Let's use the same example. Our subnet mask is 255.255.255.252, which is 11111111 11111111 11111111 11111100 when converted to binary. This subnet m= ask had two 0s. So, 22-2 =3D 2, which means there are two valid host= s in the subnet. The 2 we subtract in the formula stand for the network ID and t= he broadcast ID.
Let's say the IP address for this example is 192.168.1.0 with a /30 subn= et mask (the 255.255.255.252 subnet mask above). This network would look like this:
192.168.1.0: Network ID
192.168.1.1: One of the usable hosts=
192.168.1.2: One of the usable hosts
192.168.1.3: Broa= dcast ID
So, by using the /30 subnet mask, we've created 64 new networks with two useable hosts each.
A more experienced administrator out there might be wondering why we did= n't subtract 2 when using the networks formula. Let me clarify: The original networks formula was 2x-2, where the subtracted 2 stood for netw= orks with all 0s and all 1s. However, with modern IP stacks, you can use these networks, and it's no longer necessary to subtract them (RFC 1878).
Prior to Cisco IOS 12.0, by default, Cisco routers wouldn't allow you to configure an IP address on the all 0s network on an interface. However, you could configure this using the ip subnet-zero command in Global Configuration Mode.
Now, after IOS 12.0, the ip subnet-zero command is the default on routers. Note that this command not only allows the all 0s subnet, but it a= lso permits the all 1s subnet. And that's why you no longer have to subtract 2 = when using the network formula. For more information on this, see Cis= co's "Subnet Zero and the All-Ones Subnet" documentation.
Many CCNA test candidates want to know which subnetting formulas they sh= ould know for the CCNA exam. I researched this question, and here's what I found out: While most questions don't cover subnet 0 or subnet 1, it's still a possible test topic. However, the exam clearly states the situation and discloses whether the network in question uses the 0 or 1 subnets. So, I recommend CCNA exam candidates make sure they understand the difference and read the questions carefully.
In addition to the different classes of IP addresses (i.e., A, B, C, D, = and E), you should also be familiar with special IP addresses. Let's look at th= ree categories of special IP addresses.
Private IP addresses
Based on RFC
1918, private IP addresses are not routable on the Internet. Instead, t=
he
purpose of these addresses is for use on internal networks only. (While you=
may
receive traffic from the Internet with a private IP address as the source, =
this
is actually a spoofed source IP address.)
Here are the ranges of private IP addresses:
Automatic private IP addressing
The 169.254.0.0 network with a 255.255.0.0 (/16) subnet mask is an IP addre=
ss
range used for private IP addressing in situations when a machine is unable=
to
contact a DHCP server (RFC 3927). For example, this is a default configurati=
on
when a Windows machine can't contact a DHCP server.
Loopback network
The 127.0.0.0 network with a 255.0.0.0 (/8) subnet mask is a special IP add=
ress
range from the Class A network, which administrators can use for loopback
adapters (RFC
1166). These are internal, "virtual" adapters that represent a
network interface.
Every system has a loopback adapter, and it's usually 127.0.0.1. What ma= ny people don't realize is that 127.123.3.240 also represents their loopback adapter. The standard domain name for the loopback adapter is localhost.
The CCNA exam requires candidates to be able to figure out subnetting in longhand. However, in the real world, most network administrators use a calculator.
While you can use the Windows calculator to figure out the networks and hosts formulas, several specialized IP subnetting calculators are also available. In addition, these calculators can come in handy when you're studying for the CCNA exam and want to check your manual calculations.
Here's a selection of downloadable and online tools:
There's a wealth of information available on the Internet about subnetti= ng. Once again, here are some recommended resources:
Check out the new Cisco Routers and Switches Archive, and catch up on the most recent editions of this newsletter.
Want to learn more about router and switch management? Automatically sign up for our free Cisco Routers and Switches newsletter, delivered e= ach Friday!
David Davis has worked in the IT industry for 12 years and holds seve= ral certifications, including CCIE, MCSE+I, CISSP, CCNA, CCDA, and CCNP. He currently manages a group of systems/network administrators for a privately owned retail company and performs networking/systems consulting on a part-t= ime basis.
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