Subnetting: Plain & Simple...Part 1
by Sebastian Abbinanti on August 13th, 2010For individuals looking to delve into the complex world of networking, the word subnetting will most certainly inspire a cringe. It has been my experience, that the single most difficult concept for people to understand is why we subnet. Everyone knows that old line about running out of IP address, but the wide scale use of NAT and PAT, along with the private address space, running out of IP addresses seams less likely.
The key word is seams. Before the days of CIDR (Classless Inter-Domain Routing) all IP address were broken down into classful boundaries. That meant that the smallest network that could be assigned was a class C, consisting of 254 usable IP addresses. Every ISP would be required to assigned 254 IP address for every client, even for a home user that may only have 1 computer. This is regardless of NAT.
What You Need to Know
When it comes to subnetting, you should be able to do three things.
- You must be able to determine how many networks, of a given size, you can fit into a large, given, network.
- You must be able to determine that Network ID and Broadcast Address of a given IP address and Subnet Mask.
- You must be able to break down, or subnet, a large network into smaller networks of varying sizes.
Breaking it Down
By far the easiest of the three is the first one. When given a network, say 192.168.0.0/24, you must be able to determine how many smaller networks, say a /27, the large network can yield. What makes the first the easiest, is the fact that you use one simple formula.
Using the above example, we want to determine ho many smaller networks can be derived from the larger network. The answer is simple.
2(27-24) = 23 = 8
Stay Tuned...More to Come
Sebastian Abbinanti, CCNA