I’ve been working on my Cisco CCNA certification lately and I’ve been really getting into the practice of subnetting now that I have all of the concepts and fundamentals down pat. Here’s the bottom line, the subnet mask is your key to everything. The subnet mask rules all. There is no difference in difficulty between subnetting a class A address or a class C address. The subnet mask is the only thing that matters. Here’s what I mean:
Random IP Address: 157.142.254.117
Random Subnet Mask: 255.255.0.0
We want to know the network address and the broadcast address from this information. Here’s how I’d figure this out:
- First realize that this is NOT subnetted at all!
- The first two octets remain the same, and the second two octets default to subnet ID’s and broadcast IDs.
Network address: 157.142.0.0
Broadcast Address: 157.142.255.255
The broadcast address means that all bits in the broadcast portion are turned on, or are 1′s. Therefore, in this case, all octets after the first two (determined by the subnet mask) are 255.
Okay that was relatively easy. Let’s do something different:
Random IP Address: 21.149.6.207
Random Subnet Mask: 248.0.0.0
- First thing to do: realize that because the first octet in the subnet mask is not 255, we are subnetting it. This means we care about the first octet for our purposes. 21 in this case.
- The subnet mask says 248, and that’s a /29 in slash notation, and it is also an 8 in the binary table. 8 is our “magic number”
- Now we need to determine where our 21 fits into the binary table so we can figure out what set of usable hosts it fits into. 21 is inbetween 16 and 32 on the binary table in steps of 8 (magic number)
- So we start at 16 and count in steps of 8, the next number is 24. 21 is inbetween 16 and 24 now so we can determine our final addresses.
- The subnet ID is going to be 16 because this is the lowest network ID below 21.
- The broadcast is going to be 23 because this is the next broadcast number below 24
Subnet ID: 16.0.0.0
Broadcast: 23.255.255.255
This information on subnetting is really a reminder for me. Feel free to take what you can from it. I might clean it up later. Also, here’s a few pages that might help out as well:
Steve Kehlet’s Subnetting Quiz
Subnetting Introduction on Pantz.org
